What is the test for an independent review under People v. Wende?

We appointed counsel to represent defendant in this court. Appointed counsel filed an opening brief that stated the case and the facts but raised no specific issues and requested this court to conduct an independent review under People v. Wende (1979) 25 Cal.3d 436. We notified defendant of his right to submit written argument on his own behalf within 30 days. This period has elapsed and we have received no written argument from defendant.