nixos/mpd: allow services.mpd.dbFile to be null

This change allows users to specify an alternative database method. For
example an mpd satellite setup where another mpd on the network shares
it's database with the local instance. The `dbFile` parameter must not be
configured in that case.

nixos/modules/services/audio/mpd.nix

@@ -13,7 +13,9 @@ let
   mpdConf = pkgs.writeText "mpd.conf" ''
     music_directory     "${cfg.musicDirectory}"
     playlist_directory  "${cfg.playlistDirectory}"
-    db_file             "${cfg.dbFile}"
+    ${lib.optionalString (cfg.dbFile != null) ''
+      db_file             "${cfg.dbFile}"
+    ''}
     state_file          "${cfg.dataDir}/state"
     sticker_file        "${cfg.dataDir}/sticker.sql"
     log_file            "syslog"
@@ -126,11 +128,12 @@ in {
       };
 
       dbFile = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         default = "${cfg.dataDir}/tag_cache";
         defaultText = ''''${dataDir}/tag_cache'';
         description = ''
-          The path to MPD's database.
+          The path to MPD's database. If set to <literal>null</literal> the
+          paramter is omitted from the configuration.
         '';
       };
     };