Fix registration available tag

Check that the user has a linked attendee before
trying to check if they have completed registration.

vendor/registrasion/registrasion/templatetags/registrasion_tags.py

@@ -29,7 +29,7 @@ def registration_available(context):
         # Staff can always see registration - not necessarily all parts.
         return True
 
-    if user.attendee.completed_registration:
+    if hasattr(user, "attendee") and user.attendee.completed_registration:
         # Always show to someone who has completed registration already.
         return True