wallpapers/potd: clear `Drag.imageSource` on `dragFinished`

(cherry picked from commit a8a7dee8)

wallpapers/potd/package/contents/ui/WallpaperDelegate.qml

@@ -160,6 +160,7 @@ FocusScope {
                         });
                     } else {
                         parent.Drag.active = false;
+                        parent.Drag.imageSource = "";
                     }
                 }