AdminZulipHandler: Don't print extra traceback for logged-out users.

This is not an interesting condition worth highlighting for sysadmins.

Fixes #3963.

zerver/logging_handlers.py

@@ -38,12 +38,17 @@ class AdminZulipHandler(logging.Handler):
                 stack_trace = None
 
             try:
+                from django.contrib.auth.models import AnonymousUser
                 user_profile = request.user
+                if isinstance(user_profile, AnonymousUser):
+                    user_full_name = None
+                    user_email = None
+                else:
                     user_full_name = user_profile.full_name
                     user_email = user_profile.email
             except Exception:
+                # Unexpected exceptions here should be handled gracefully
                 traceback.print_exc()
-                # Error was triggered by an anonymous user.
                 user_full_name = None
                 user_email = None