Comment 4 for bug 1703105

Log cleared by percona's systemd helper:
https://github.com/percona/percona-server/blob/5.6/build-ps/rpm/mysql-systemd#L58

install -m 0640 -omysql -gmysql /dev/null /var/log/mysqld.log

Upstream is not affected, because it uses ExecStartPre=/usr/bin/mysql-systemd-start pre
+ log=/var/log/mysqld.log
+ '[' -e /var/log/mysqld.log ']'
+ chmod 0640 /var/log/mysqld.log
+ chown mysql:mysql /var/log/mysqld.log

PS 5.7 is not affected, because it checks -e /var/log/mysqld.log
https://github.com/percona/percona-server/blob/5.7/scripts/systemd/mysqld_pre_systemd.in#L62