domain should default to a blank string, not "unset"

[James Simpson]

Seen in a recent deployment of userdir-ldap:

ud-replicate fails, unable to find the requested file:
makedb: cannot open input file `juju-751555-prod-error-tracker-cassandra-0/passwd.tdb': No such file or directory

/var/lib/misc/thishost is a symlink to $(hostname).None: "lrwxrwxrwx 1 root root 47 Jul 19 09:49 thishost -> juju-751555-prod-error-tracker-cassandra-0.None"

This can be seen in the Juju unit logs:

2023-07-19 09:49:07 DEBUG unit.userdir-ldap/9.juju-log server.go:325 setup_udldap, config: {...'ciphers': '<email address hidden>,<email address hidden>,<email address hidden>', 'domain': None, ...}

Note that this is not a string value - it's showing up as "None", not "'None'".

juju config shows that this value is "unset":
  domain:
    description: Fallback domain when none present. This is mostly to work around
      MAAS's failure to add DNS for LXC containers - LP#1274947.
    source: unset
    type: string

And after setting it explicitly to an empty string, things start working:
  domain:
    description: Fallback domain when none present. This is mostly to work around
      MAAS's failure to add DNS for LXC containers - LP#1274947.
    source: user
    type: string
    value: ""

2023-09-20 04:57:18 DEBUG unit.userdir-ldap/9.juju-log server.go:325 setup_udldap, config: {'...'ciphers': '<email address hidden>,<email address hidden>,<email address hidden>', 'domain': '',...}

ubuntu@juju-751555-prod-error-tracker-cassandra-0:~$ sudo ud-replicate
receiving file list ... done
...
sent 13,031 bytes received 9,258 bytes 8,915.60 bytes/sec
total size is 1,494,751 speedup is 67.06

[Eric Chen (eric-chen)]

Moved to https://github.com/canonical/charm-userdir-ldap/issues/20
Close this one.