Add default parameter to CommandTree.sync

I'm not sure how I feel about this, since explicitly passing
guild=None seems better.

discord/app_commands/tree.py

@@ -543,7 +543,7 @@ class CommandTree(Generic[ClientT]):
 
         return decorator
 
-    async def sync(self, *, guild: Optional[Snowflake]) -> List[AppCommand]:
+    async def sync(self, *, guild: Optional[Snowflake] = None) -> List[AppCommand]:
         """|coro|
 
         Syncs the application commands to Discord.

discord/guild.py

@@ -3344,7 +3344,7 @@ class Guild(Hashable):
         -------
         ClientException
             The members intent is not enabled.
-        
+
         Returns
         --------
         Optional[List[:class:`Member`]]