tag/Set: do AlbumArtist/Artist fallback only if AlbumArtist is not disabled

On "list albumartist", songs that have no AlbumArtist tag will use the
Artist tag.  However, if AlbumArtist is disabled via
"metadata_to_use", the TagBuilder::AddItem() call is ignored, and
PrintUniqueTag() attempts to print a nullptr string.

This commit fixes the problem by attempting the fallback only if
AlbumArtist is not disabled.

NEWS

@@ -1,4 +1,6 @@
 ver 0.19.3 (not yet released)
+* protocol
+  - fix "(null)" result string to "list" when AlbumArtist is disabled
 * database
   - upnp: fix breakage due to malformed URIs
 * decoder

src/tag/Set.cxx

@@ -19,6 +19,7 @@
 
 #include "Set.hxx"
 #include "TagBuilder.hxx"
+#include "TagSettings.h"
 
 #include <assert.h>
 
@@ -109,6 +110,7 @@ TagSet::InsertUnique(const Tag &tag,
 
 	if (!CheckUnique(type, tag, type, group_mask) &&
 	    (type != TAG_ALBUM_ARTIST ||
+	     ignore_tag_items[TAG_ALBUM_ARTIST] ||
 	     /* fall back to "Artist" if no "AlbumArtist" was found */
 	     !CheckUnique(type, tag, TAG_ARTIST, group_mask)))
 		InsertUnique(tag, type, nullptr, group_mask);