Ensure no NULL tags are set

Because `mpd_song_get_tag` from libmpdclient can return NULL, verify the
value of tag is valid. Otherwise, set a default string of "N/A". Also
adds configuration to specify what this default string should be.

resources/config

@@ -30,6 +30,7 @@
         "format": "{stateIcon} {consumeIcon}{randomIcon}{repeatIcon}{singleIcon}{artist} - {album} - {title} ({elapsedTime:%M:%S}/{totalTime:%M:%S}) ",
         "format-disconnected": "Disconnected ",
         "format-stopped": "{consumeIcon}{randomIcon}{repeatIcon}{singleIcon}Stopped ",
+        "unknown-tag": "N/A",
         "interval": 2,
         "consume-icons": {
             "on": " "