Fixed a bug that prevented the display of multiple empty playlists

Turned out this bug was already fixed two years ago but unfortunately
it made its way back again. It was solved in #9642 but back then another
method was used for querying playlists from db (for add playlist dialog) then in
ef4a623 was introduced another method
which had the same problem as fixed in #9642 and which eventually replaced
original method in code for querying playlists from db (for add playlist dialog)

Author: dustdfg

app/src/main/java/org/schabi/newpipe/database/playlist/dao/PlaylistStreamDAO.kt

@@ -68,6 +68,11 @@ interface PlaylistStreamDAO : BasicDAO<PlaylistStreamEntity> {
     )
     fun getOrderedStreamsOf(playlistId: Long): Flowable<MutableList<PlaylistStreamEntry>>
 
+    // If a playlist has no streams, there won’t be any rows in the **playlist_stream_join** table
+    // that have a foreign key to that playlist. Thus, the **playlist_id** will not have a
+    // corresponding value in any rows of the join table. So, if you group by the **playlist_id**,
+    // only playlists that contain videos are grouped and displayed. Look at #9642 #13055
+
     @Transaction
     @Query(
         """
@@ -103,6 +108,11 @@ interface PlaylistStreamDAO : BasicDAO<PlaylistStreamEntity> {
     )
     fun getStreamsWithoutDuplicates(playlistId: Long): Flowable<MutableList<PlaylistStreamEntry>>
 
+    // If a playlist has no streams, there won’t be any rows in the **playlist_stream_join** table
+    // that have a foreign key to that playlist. Thus, the **playlist_id** will not have a
+    // corresponding value in any rows of the join table. So, if you group by the **playlist_id**,
+    // only playlists that contain videos are grouped and displayed. Look at #9642 #13055
+
     @Transaction
     @Query(
         """
@@ -118,7 +128,7 @@ interface PlaylistStreamDAO : BasicDAO<PlaylistStreamEntity> {
         LEFT JOIN streams
         ON streams.uid = stream_id AND :streamUrl = :streamUrl
 
-        GROUP BY playlist_id
+        GROUP BY playlists.uid
         ORDER BY display_index, name
         """
     )