Fix the incorrect thumbnails for albums issue (#1198)

* The fix with test

* Fix test's foreign key issue

* 2 more test cases

* Fix the empty album test expectation

* Return Nil for no thumbnail case instead of an empty album, as expected by frontend. More tests for thumbnail priority choice

* Fix failed tests

---------

Co-authored-by: Konstantin Koval

api/graphql/models/album.go

@@ -98,7 +98,6 @@ func (a *Album) Thumbnail(db *gorm.DB) (*Media, error) {
 			INNER JOIN sub_albums ON children.parent_album_id = sub_albums.id
 		)
 		SELECT * FROM media
-		INNER JOIN media_urls ON media_urls.media_id = media.id
 		WHERE media.album_id IN (SELECT id FROM sub_albums)
 		LIMIT 1
 	`
@@ -107,5 +106,9 @@ func (a *Album) Thumbnail(db *gorm.DB) (*Media, error) {
 		return nil, err
 	}
 
+	if media.ID == 0 {
+		return nil, nil // Return nil for empty albums
+	}
+
 	return &media, nil
 }