Check for Unset type in Emoji.custom (#60)

* Check for Unset type in Emoji.custom

* Typecase id value to bool

in the case where id is Unset this will call the builtin
__bool__ which returns False. This is not ideal beacuse
an Id of 0 will mean the emoji is custom, but to fix would
require a deeper refactor of the Field logic. Lets put a pin
in it.

* Remove unused import

disco/types/message.py

@@ -43,7 +43,7 @@ class Emoji(SlottedModel):
 
     @cached_property
     def custom(self):
-        return self.id is not None
+        return bool(self.id)
 
     def __eq__(self, other):
         if isinstance(other, Emoji):