Inserts a "None" option at the top of the ajax list if the field is nullable

If I have a model field that is nullable, 

    group = peewee.ForeignKeyField(Group, null=True, db_index=True, rules=Rules(api=ApiRule(edit=False)))

Along with this in my model admin

    foreign_key_lookups = {'group': 'name'}

I get the nice ajax popup, but I can't null out the field if I have previously set a value.

This change adds a None option to the top of the ajax popup.