function string(fallback, allow_empty = true) {
	return validator(fallback, (input, keypath) => {
		if (typeof input !== 'string') {
			throw_error(`${keypath} should be a string, if specified`);
		}

		if (!allow_empty && input === '') {
			throw_error(`${keypath} cannot be empty`);
		}

		return input;
	});
}
@param {string | undefined} fallback
@param {boolean} allow_empty
@returns {Validator}