Skip choices for nullable argument in RequestParser

[adzhurinskij]

Hi.

It's fix popular case with arguments which can be choices or nullable.

parser = reqparse.RequestParser()
parser.add_argument('description', type=str, required=False, store_missing=False, nullable=True)
parser.add_argument('location_id', type=int, required=False, store_missing=False, nullable=True, choices=db.session.query(Locations.id).filter(Locations.deleted_at.is_(None)).values())
args = parser.parse_args(strict=True)

[andreixk]

@adzhurinskij You need to modify unit tests accordingly in order to ensure the checks are passing.