allow searching for albums with an empty tag

tfing wrote:
> I have quite some files with an empty album tag as they do not come
> from a particular album.
>
> If I want to look for those files and browse them, this happens:
> :: nc localhost 6600
> OK MPD 0.12.0
> find album ""
> ACK [2@0] {find} too few arguments for "find"
>
> I'd like to be able to browse those files in a client like gmpc.
> So these 2 items would have to be developed:
> - list album should report that some files have an empty tag
> - it should be possible to search for an empty tag with the find command

Patch-by: Marc Pavot
ref: http://musicpd.org/mantis/view.php?id=464

src/dbUtils.c

@@ -295,8 +295,10 @@ static void visitTag(struct client *client, struct strset *set,
 	for (i = 0; i < tag->numOfItems; i++) {
 		if (tag->items[i]->type == tagType) {
 			strset_add(set, tag->items[i]->value);
+			return;
 		}
 	}
+	strset_add(set, "");
 }
 
 struct list_tags_data {