Set Parent to current query Item if ParentId is empty (#680)

**Changes**
Fixed a faulty change in logic that occurred during the Christmas GPL nightmare.

I stepped through Emby 3.5.2 and stumbled over a difference in the query between Jellyfin and Emby, where in Emby, query.ParentId was set to the item itself. So I did a little digging in the old GPLv2 source and found this https://github.com/MediaBrowser/Emby/blob/3.4.1.8/MediaBrowser.Controller/Entities/Folder.cs#L720

**Issues**
Fixes #673

MediaBrowser.Controller/Entities/Folder.cs

@@ -644,12 +644,9 @@ namespace MediaBrowser.Controller.Entities
                 return PostFilterAndSort(items, query, true);
             }
 
-            if (!(this is UserRootFolder) && !(this is AggregateFolder))
+            if (!(this is UserRootFolder) && !(this is AggregateFolder) && query.ParentId == Guid.Empty)
             {
-                if (!query.ParentId.Equals(Guid.Empty))
-                {
-                    query.Parent = this;
-                }
+                query.Parent = this;
             }
 
             if (RequiresPostFiltering2(query))