Bug: 事件响应器异常
**描述问题：**

任意事件响应器
```
from nonebot import on_message
message = on_message()
```
对于任何消息都会引发异常
```
01-01 14:57:55 [DEBUG] nonebot | CQHTTP | Event Parser Error
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\multiprocessing\\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\multiprocessing\\spawn.py", line 129, in _main
    return self._bootstrap(parent_sentinel)
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\multiprocessing\\process.py", line 315, in _bootstrap
    self.run()
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\multiprocessing\\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\\Users\\...\\PycharmProjects\\nonebot2\\venv\\lib\\site-packages\\uvicorn\\subprocess.py", line 62, in subprocess_started
    target(sockets=sockets)
  File "C:\\Users\\...\\PycharmProjects\\nonebot2\\venv\\lib\\site-packages\\uvicorn\\main.py", line 390, in run
    loop.run_until_complete(self.serve(sockets=sockets))
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\asyncio\\base_events.py", line 603, in run_until_complete
    self.run_forever()
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\asyncio\\base_events.py", line 570, in run_forever
    self._run_once()
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\asyncio\\base_events.py", line 1859, in _run_once
    handle._run()
  File "C:\\Users\\...\\AppData\\Local\\Programs\\Python\\Python38\\lib\\asyncio\\events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
> File "C:\\Users\\...\\PycharmProjects\\nonebot2\\venv\\lib\\site-packages\\nonebot\\adapters\\cqhttp\\bot.py", line 307, in handle_message
    event = model.parse_obj(message)
  File "pydantic\\main.py", line 520, in pydantic.main.BaseModel.parse_obj
    return cls(**obj)
  File "pydantic\\main.py", line 362, in pydantic.main.BaseModel.__init__
    raise validation_error
pydantic.error_wrappers.ValidationError: 1 validation error for GroupMessageEvent
anonymous
  none is not an allowed value (type=type_error.none.not_allowed)
```

**环境信息：**

 - OS: Windows 10 20H2 19042.685
 - Python Version: 3.8
 - Nonebot Version: 2.0.0a8 (从2.0.0a7升级)


diff --git a/nonebot/adapters/cqhttp/event.py b/nonebot/adapters/cqhttp/event.py
index 5d1cdb2f..39e78943 100644
--- a/nonebot/adapters/cqhttp/event.py
+++ b/nonebot/adapters/cqhttp/event.py
@@ -184,7 +184,7 @@ class GroupMessageEvent(MessageEvent):
     __event__ = "message.group"
     message_type: Literal["group"]
     group_id: int
-    anonymous: Anonymous
+    anonymous: Optional[Anonymous] = None
 
     @overrides(Event)
     def get_event_description(self) -> str:
['nonebot/adapters/cqhttp/event.py']