Fix response_type_detector: no voice for None

pqai_agent/response_type_detector.py

@@ -75,6 +75,8 @@ class ResponseTypeDetector:
 
     @staticmethod
     def if_message_suitable_for_voice(message):
+        if not message:
+            return False
         # 使用语音的文字不适合过长
         if len(message) > 50:
             return False