Fix UnboundLocalError in generate_gate when generator yields nothing

Initialize x before the loop to prevent UnboundLocalError if
generate_stream_gate yields no items.

Fixes #3786

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Chessing234

fastchat/serve/model_worker.py

@@ -144,6 +144,7 @@ def generate_stream_gate(self, params):
             yield json.dumps(ret).encode() + b"\0"
 
     def generate_gate(self, params):
+        x = b"{}\0"
         for x in self.generate_stream_gate(params):
             pass
         return json.loads(x[:-1].decode())