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.

include/modules/mpd.hpp

@@ -15,6 +15,7 @@ class MPD : public ALabel {
 
  private:
   std::thread periodic_updater();
+  std::string getTag(mpd_tag_type type, unsigned idx = 0);
   void        setLabel();
   std::string getStateIcon();
   std::string getOptionIcon(std::string optionName, bool activated);