`UnboundLocalError` in `generate_gate` if generator yields nothing

[MrBMRBLX]

In fastchat/serve/model_worker.py:

def generate_gate(self, params):
    for x in self.generate_stream_gate(params):
        pass
    return json.loads(x[:-1].decode())

If the generator yields nothing (e.g., an exception before the first yield), x is never defined and this crashes with UnboundLocalError.