Fix Mass ungrouping

lib/src/screens/event.dart

@@ -536,7 +536,7 @@ class _EventFightList extends HookConsumerWidget {
                                   fight.kampfleiter.id,
                                   fight.blueTeam,
                                   fight.redTeam,
-                                  group);
+                                  group.isEmpty ? null : group);
                             }
                           }
                           update();