Bug: @on_notice(sth).send() reply a group message when receive notice in private chat
**描述问题：**

私聊注册一个on_notice以后（比如戳一戳），生成的PokeNotifyEvent中存在attr 'group_id' = None；
但是在cqhttp的adapter中（_见nonebot/adapters/cqhttp/bot.py function Bot.send()_)，send只会检测group_id是否存在于字典中，而不检测它是否为None，因此会把这条私聊的notice作为群聊消息返回。
详细代码如下：
```python
        if "message_type" not in params:
            if "group_id" in params:
                params["message_type"] = "group"
            elif "user_id" in params:
                params["message_type"] = "private"
            else:
                raise ValueError("Cannot guess message type to reply!")
```
（实际上改动一下if statement就好？但我不敢提pr，没怎么读过代码）

**如何复现？**

见上文。

**期望的结果**

@on_notice(sth).send() 应该发送私聊消息

**环境信息：**

 - OS: OS Independent
 - Python Version: 3.8
 - Nonebot Version: 2.0.0a8post1

**截图**

暂无
diff --git a/nonebot/adapters/cqhttp/bot.py b/nonebot/adapters/cqhttp/bot.py
index aa2783f3..1aff6d5d 100644
--- a/nonebot/adapters/cqhttp/bot.py
+++ b/nonebot/adapters/cqhttp/bot.py
@@ -421,19 +421,19 @@ class Bot(BaseBot):
         message = escape(message) if isinstance(message, str) else message
         msg = message if isinstance(message, Message) else Message(message)
 
-        at_sender = at_sender and hasattr(event, "user_id")
+        at_sender = at_sender and getattr(event, "user_id", None)
 
         params = {}
-        if hasattr(event, "user_id"):
+        if getattr(event, "user_id", None):
             params["user_id"] = getattr(event, "user_id")
-        if hasattr(event, "group_id"):
+        if getattr(event, "group_id", None):
             params["group_id"] = getattr(event, "group_id")
         params.update(kwargs)
 
         if "message_type" not in params:
-            if "group_id" in params:
+            if params.get("group_id", None):
                 params["message_type"] = "group"
-            elif "user_id" in params:
+            elif params.get("user_id", None):
                 params["message_type"] = "private"
             else:
                 raise ValueError("Cannot guess message type to reply!")
['nonebot/adapters/cqhttp/bot.py']