Fix: Ensure `user_oauth_groups` defaults to an empty list to prevent TypeError

When the OAuth groups claim does not yield a list, `user_oauth_groups` was previously
set to None, causing a TypeError during membership checks. Changed this default to
an empty list (`[]`) to ensure the variable is always iterable, preventing errors
for non-admin users while logging in.

This fix ensures stability in the `update_user_groups` function.

backend/open_webui/utils/oauth.py

@@ -146,7 +146,7 @@ class OAuthManager:
             nested_claims = oauth_claim.split(".")
             for nested_claim in nested_claims:
                 claim_data = claim_data.get(nested_claim, {})
-            user_oauth_groups = claim_data if isinstance(claim_data, list) else None
+            user_oauth_groups = claim_data if isinstance(claim_data, list) else []
 
         user_current_groups: list[GroupModel] = Groups.get_groups_by_member_id(user.id)
         all_available_groups: list[GroupModel] = Groups.get_groups()