fix: add None return for uncaught conditional

Author: CasperGN

dapr_agents/types/message.py

@@ -240,6 +240,7 @@ def get_tool_calls(self) -> Optional[List[ToolCall]]:
             return self.tool_calls
         if isinstance(self.tool_calls, ToolCall):
             return [self.tool_calls]
+        return None
 
     def has_tool_calls(self) -> bool:
         """