Mapper: _get_music_directory_utf8() may return nullptr

If no music_directory is configured, return nullptr instead of an
empty string.  This fixes a crash when db_file is configured without
music_directory.

src/Mapper.hxx

@@ -41,7 +41,8 @@ mapper_init(AllocatedPath &&music_dir, AllocatedPath &&playlist_dir);
 void mapper_finish(void);
 
 /**
- * Return the absolute path of the music directory encoded in UTF-8.
+ * Return the absolute path of the music directory encoded in UTF-8 or
+ * nullptr if no music directory was configured.
  */
 gcc_const
 const char *