fix(commands): discord.Attachment optional arg

This commit fixes an issue with the discord.Attachment argument raising the MissingRequiredArgument when used like this:
arg: discord.Attachment = None
when no attachment is provided to the command.

discord/ext/commands/core.py

@@ -709,6 +709,8 @@ class Command(_BaseCommand, Generic[CogT, P, T]):
 
         # Try to detect Optional[discord.Attachment] or discord.Attachment special converter
         if converter is discord.Attachment:
+            if attachments.is_empty():
+                return None if param.default is param.empty else param.default
             try:
                 return next(attachments)
             except StopIteration: