If mariadb-service-convert is run and the user variable is unset then
this sets `User=` in `[Service]`, which then tries to run mariadb as
root, which in-turn fails. This only happens when mysqld_safe is missing
which is all the time now. So don't set `User=` if there is no user variable.

Reviewer: Sergei Golubchik <serg@mariadb.org> (in PR #2382)