nixos/postgresqlBackup: allow defining multiple times to start at

Or … none! Because forcing a string always results in an OnCalender=
setting, but an empty string leads to an empty value.

>  postgresqlBackup-hass.timer: Timer unit lacks value setting. Refusing.

or

> postgresqlBackup-miniflux.timer: Cannot add dependency job, ignoring: Unit postgresqlBackup-miniflux.timer has a bad unit file setting.

I require the postgresqlBackup in my borgbackup unit, so I don't
strictly need the timer and could previously set it to an empty list.

nixos/modules/services/backup/postgresql-backup.nix

@@ -48,7 +48,7 @@ in {
 
       startAt = mkOption {
         default = "*-*-* 01:15:00";
-        type = types.str;
+        type = with types; either (listOf str) str;
         description = ''
           This option defines (see <literal>systemd.time</literal> for format) when the
           databases should be dumped.