Allow TTL to be empty (to use the default Zone TTL)

[JPustkuchen]

At Hetzner DNS if TTL is empty, the Zone default TTL is used. This currently isn't supported here,

Currently, when updating a record value with no explicit TTL, the following exception is thrown:

PHP Fatal error: Uncaught MehrIt\HetznerDnsApi\Exceptions\ValidationException: Record ttl must not be empty in /hetzner-api-tools/hetzner-dns-api/src/Concerns/Records.php:154

because of the following validations:

/**
 * Validates the given record
 * @param Record $record The record
 * @throws ValidationException
 */
protected function validateRecord(Record $record): void {

	if (trim($record->getName()) === '')
		throw new ValidationException('Record name must not be empty');
	if (trim($record->getTtl()) === '')
		throw new ValidationException('Record ttl must not be empty');
	if (trim($record->getType()) === '')
		throw new ValidationException('Record type must not be empty');
	if (trim($record->getValue()) === '')
		throw new ValidationException('Record value must not be empty');
	if (trim($record->getZoneId()) === '')
		throw new ValidationException('Record zoneId must not be empty');
}

where

if (trim($record->getTtl()) === '')
    throw new ValidationException('Record ttl must not be empty');

should simply be removed to make it work.

Empty TTL is supported and a nice and helpful feature!