Confirmed with MariaDB 10.0.1, If @@default_master_connection will be null, it will not show result in "show slave status \G"

mysql> \s
--------------
mysql  Ver 14.14 Distrib 5.6.22-71.0, for debian-linux-gnu (x86_64) using  EditLine wrapper
...
Server version:		5.5.5-10.0.15-MariaDB-log MariaDB Server

mysql> select @@default_master_connection;
+-----------------------------+
| @@default_master_connection |
+-----------------------------+
|                             |
+-----------------------------+
1 row in set (0.00 sec)

mysql> show slave status \G
Empty set (0.00 sec)

mysql> show all slaves status\G
*************************** 1. row ***************************
              Connection_name: master1
              Slave_SQL_State: Slave has read all relay log; waiting for the slave I/O thread to update it
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 127.0.0.1
                  Master_User: rsandbox
                  Master_Port: 24025
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 326
               Relay_Log_File: mysql_sandbox24026-relay-bin-master1.000004
                Relay_Log_Pos: 535
        Relay_Master_Log_File: mysql-bin.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 326
              Relay_Log_Space: 853
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 1
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
                   Using_Gtid: No
                  Gtid_IO_Pos: 
         Retried_transactions: 0
           Max_relay_log_size: 1073741824
         Executed_log_entries: 6
    Slave_received_heartbeats: 0
       Slave_heartbeat_period: 1800.000
               Gtid_Slave_Pos: 0-1-12
*************************** 2. row ***************************
              Connection_name: master2
              Slave_SQL_State: Slave has read all relay log; waiting for the slave I/O thread to update it
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 127.0.0.1
                  Master_User: rsandbox
                  Master_Port: 24027
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 312
               Relay_Log_File: mysql_sandbox24026-relay-bin-master2.000004
                Relay_Log_Pos: 535
        Relay_Master_Log_File: mysql-bin.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 312
              Relay_Log_Space: 853
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 102
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
                   Using_Gtid: No
                  Gtid_IO_Pos: 
         Retried_transactions: 0
           Max_relay_log_size: 1073741824
         Executed_log_entries: 6
    Slave_received_heartbeats: 0
       Slave_heartbeat_period: 1800.000
               Gtid_Slave_Pos: 0-1-12
2 rows in set (0.00 sec)


But if @@default_master_connection is set. i.e 

mysql> select @@default_master_connection;
+-----------------------------+
| @@default_master_connection |
+-----------------------------+
| master1                     |
+-----------------------------+
1 row in set (0.00 sec)

mysql> show slave status \G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 127.0.0.1
                  Master_User: rsandbox
                  Master_Port: 24025
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 326
               Relay_Log_File: mysql_sandbox24026-relay-bin-master1.000004
                Relay_Log_Pos: 535
        Relay_Master_Log_File: mysql-bin.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 326
              Relay_Log_Space: 853
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 1
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
                   Using_Gtid: No
                  Gtid_IO_Pos: 
1 row in set (0.00 sec)

mysql> 

Then show slave status will show the output but only for default master.