[commands] Fix lambda converters in non-module contexts.

Not sure why anyone would do this but might as well fix it.

discord/ext/commands/core.py

@@ -325,7 +325,7 @@ class Command(_BaseCommand):
         except AttributeError:
             pass
         else:
-            if module.startswith('discord.') and not module.endswith('converter'):
+            if module is not None and (module.startswith('discord.') and not module.endswith('converter')):
                 converter = getattr(converters, converter.__name__ + 'Converter')
 
         try: