puppet: Only restart PostgreSQL if there is a datadir to read.

This is specifically to support Kandra's `setup_disks`, which stops
PostgreSQL and moves the data directory out of the way while mounting
a new disk; restarting PostgreSQL would fail in this state.  We
install secrets and re-run puppet to finish bootstrapping the
database, all of which expects the PostgreSQL server to be stopped
anyways.

puppet/zulip/manifests/profile/postgresql.pp

@@ -77,5 +77,6 @@ class zulip::profile::postgresql {
     require     => $require,
     refreshonly => true,
     subscribe   => [ File[$postgresql_conf_file] ],
+    onlyif      => "test -d ${zulip::postgresql_base::postgresql_datadir}",
   }
 }