Fixed a NoneType error when the callback is not defined in project settings.

Author: diegocastrum

django_celery_results/settings.py

@@ -5,7 +5,6 @@
 
 def get_callback_function(settings_name, default=None):
     """Return the callback function for the given settings name."""
-
     callback = getattr(settings, settings_name, None)
     if not callback:
         return default
@@ -22,12 +21,11 @@ def get_callback_function(settings_name, default=None):
 
 
 def get_task_props_extension(request, task_props):
-    """Extend the task properties with custom properties to fill custom models."""
-
-    task_props_extension = extend_task_props_callback(request, task_props) or {}
-    if task_props_extension is None:
+    """Extend the task properties with custom props to fill custom models."""
+    if not extend_task_props_callback:
         return {}
 
+    task_props_extension = extend_task_props_callback(request, task_props) or {}
     if not isinstance(task_props_extension, Mapping):
         raise ImproperlyConfigured(
             "CELERY_RESULTS_EXTEND_TASK_PROPS_CALLBACK must return a Mapping "