Fix is_untis_primary_source helper method

aleksis/apps/untis/util/mysql/util.py

@@ -169,4 +169,6 @@ def move_weekday_to_range(time_periods_ref: dict, weekday: int) -> int:
 def is_untis_primary_source(model: Type[Model]) -> bool:
     """Check if Untis is primary source for the model."""
     allowed_content_types = get_site_preferences()["untis_mysql__data_types_primary_source"]
-    return ContentType.objects.get_for_model(model) in allowed_content_types
+    if allowed_content_types:
+        return ContentType.objects.get_for_model(model) in allowed_content_types
+    return False
\ No newline at end of file