fix(platform): prevent 'NoneType' object is not iterable in _outline_chain and set_result (#3103)

Guard against cases where message chain is None during pipeline execution. This change enhances error-resilience for logging and processing message chains.

- Updated AstrMessageEvent._outline_chain to return an empty string when input chain is None
- Updated AstrMessageEvent.set_result to ensure result.chain is always at least an empty list

This prevents TypeError when result.chain or chain is unexpectedly None, improving pipeline stability when handling external plugins or corner cases.

Co-authored-by: engine-labs-app[bot] <140088366+engine-labs-app[bot]@users.noreply.github.com>
Co-authored-by: cto-new[bot] <140088366+cto-new[bot]@users.noreply.github.com>

astrbot/core/platform/astr_message_event.py

@@ -92,8 +92,10 @@ class AstrMessageEvent(abc.ABC):
         """
         return self.message_str
 
-    def _outline_chain(self, chain: List[BaseMessageComponent]) -> str:
+    def _outline_chain(self, chain: Optional[List[BaseMessageComponent]]) -> str:
         outline = ""
+        if not chain:
+            return outline
         for i in chain:
             if isinstance(i, Plain):
                 outline += i.text
@@ -265,6 +267,9 @@ class AstrMessageEvent(abc.ABC):
         """
         if isinstance(result, str):
             result = MessageEventResult().message(result)
+        # 兼容外部插件或调用方传入的 chain=None 的情况，确保为可迭代列表
+        if isinstance(result, MessageEventResult) and result.chain is None:
+            result.chain = []
         self._result = result
 
     def stop_event(self):