Fix crash on reaction remove.

Discord can sometimes send integer 0 as the emoji id instead of
null to signify a non-custom emoji, which was causing a crash due
to a 'is not None' check assuming the reaction was for an emoji
with id 0. Probably a discord bug, but preferable to handle here
rather than crash users.

discord/state.py

@@ -688,7 +688,7 @@ class ConnectionState:
     def _get_reaction_emoji(self, **data):
         id = data['id']
 
-        if id is None:
+        if not id:
             return data['name']
 
         for server in self.servers: