def extract_type(item):
    """Extract item possible types from jsonschema definition.
    >>> extract_type({'type': 'string'})
    ['string']
    >>> extract_type(None)
    []
    >>> extract_type({})
    []
    >>> extract_type({'type': ['string', 'null']})
    ['string', 'null']
    """
    if not item or "type" not in item:
        return []
    type_ = item["type"]
    if not isinstance(type_, list):
        type_ = [type_]
    return type_