Fix IDE warnings (#12085)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/apps/user_app.py

@@ -660,7 +660,7 @@ def user_register(user_id, user):
     tenant_llm = get_init_tenant_llm(user_id)
 
     if not UserService.save(**user):
-        return
+        return None
     TenantService.insert(**tenant)
     UserTenantService.insert(**usr_tenant)
     TenantLLMService.insert_many(tenant_llm)