fix: fix 'NoneType' object does not support item assignment error (#2525)

### Problem Description
When `context` is `None`, it should not be used for assignment operations.

### Solution
Adjusted the code logic to ensure that `context` is not `None` before performing any item assignment.

channel/chat_channel.py

@@ -146,6 +146,7 @@ class ChatChannel(Channel):
                 elif context["origin_ctype"] == ContextType.VOICE:  # 如果源消息是私聊的语音消息，允许不匹配前缀，放宽条件
                     pass
                 else:
+                    logger.info("[chat_channel]receive single chat msg, but checkprefix didn't match")
                     return None
             content = content.strip()
             img_match_prefix = check_prefix(content, conf().get("image_create_prefix",[""]))