Use file name for unknown tag title retrieval.

When taglib get empty or null title from file/album, use file name or
folder name as the tag_title field. It could be confuse to user, but at
leaset it is better than tracks/albums can't be search in ID3 mode API.

scanner/scanner.go

@@ -465,8 +465,9 @@ func populateTrack(tx *db.DB, album *db.Album, track *db.Track, trags tagcommon.
 	track.Size = size
 	track.AlbumID = album.ID
 
-	track.TagTitle = trags.Title()
-	track.TagTitleUDec = decoded(trags.Title())
+	tagTitle := tagcommon.MustTitle(trags)
+	track.TagTitle = tagTitle
+	track.TagTitleUDec = decoded(tagTitle)
 	track.TagTrackArtist = tagcommon.MustArtist(trags)
 	track.TagTrackNumber = trags.TrackNumber()
 	track.TagDiscNumber = trags.DiscNumber()