Only call success_callback if it's not None

It could also be called as a fire-and-forget update thing.

Contributes to issue CURA-8539.

cura/OAuth2/AuthorizationHelpers.py

@@ -153,6 +153,7 @@ class AuthorizationHelpers:
                 failed_callback()
             return
 
+        if success_callback is not None:
             success_callback(UserProfile(
                 user_id = profile_data["user_id"],
                 username = profile_data["username"],