Permit fetching undefined values also if the type is specified.

boss_db.erl permits `undefined` to be written to a database, no matter what type is specified (in `validate_record_types/1`), but in boss_record_lib.erl, in `convert_value_to_type/2`, it is not allowed if the type is specified. This means that `undefined` can be written to the database, but not fetched back, if the type is specified. This PR supersedes #200.