[1.8.x] Updated recommendation for testing keyword arg in custom fields.

Backport of dbabf43920 from master

docs/topics/migrations.txt

@@ -284,7 +284,7 @@ You can't modify the number of positional arguments in an already migrated
 custom field without raising a ``TypeError``. The old migration will call the
 modified ``__init__`` method with the old signature. So if you need a new
 argument, please create a keyword argument and add something like
-``assert kwargs.get('argument_name') is not None`` in the constructor.
+``assert 'argument_name' in kwargs`` in the constructor.
 
 .. _using-managers-in-migrations: