nixos/dovecot: Fix createMailUser implementation

This option got introduced in 79044995


and it didn't check whether mailUser and mailGroup are null, which they
are by default.

Now we're only creating the user if createMailUser is set in conjunction
with mailUser and the group if mailGroup is set as well.

I've added a NixOS VM test so that we can verify whether dovecot works
without any additional options set, so it serves as a regression test
for issue #29466 and other issues that might come up with future changes
to the Dovecot service.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fixes: #29466
Cc: @qknight, @abbradar, @ixmatus, @siddharthist