[celery#305]: Added a default callable to get_callback_function

`get_callback_function()` gets a default callback as an arg
returning explicitely an empty dict.

`get_callback_function()` raises an `ImproperlyConfigured`
exception when the callback is not callable.

---
Resolves celery#305

Fixes celery#314

django_celery_results/backends/database.py

@@ -129,6 +129,8 @@ def _store_result(
             self._get_extended_properties(request, traceback)
         )
 
+        # TODO: Wrap this and make some sanity checks to complain the Mapping
+        # protocol.
         task_props.update(
             extend_task_props_callback(request, dict(task_props)))
 

django_celery_results/settings.py

@@ -1,16 +1,19 @@
 from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
 
 
 def get_callback_function(settings_name, default=None):
     """Return the callback function for the given settings name."""
 
     callback = getattr(settings, settings_name, None)
-    if callback is None:
+    if not callback:
         return default
 
-    if callable(callback):
-        return callback
+    if not callable(callback):
+        raise ImproperlyConfigured(f"{settings_name} must be callable.")
+
+    return callback
 
 extend_task_props_callback = get_callback_function(
-    "CELERY_RESULTS_EXTEND_TASK_PROPS_CALLBACK"
+    "CELERY_RESULTS_EXTEND_TASK_PROPS_CALLBACK", dict
 )