[php-src] Issue #8217: new DateTime("@".$timestamp) should not crash (throws fatal error) with empty timestamp

Issue: https://github.com/php/php-src/issues/8217 Author: PepeLopez ### Description When creating a DateTime object with @ + timestamp (e.g. @123987139) the date is forced to UTC. That works fine. **Working Code:** $timestamp =123987139; $date = new DateTime("@".$timestamp); // @ Forcing to UTC However, if the timestamp is empty, php will throw an fatal error like "PHP Fatal error: Uncaught Exception: DateTime::__construct(): Failed to parse time string (@) at position 0 (@): Unexpected character in ..." **Non-working Code:** unset($timestamp) ; $date = new DateTime("@".$timestamp); // @ Forcing to UTC **Actual behaviour:** Script throws fatal error, resulting in error 500, whole script stops **Desired behavior:** Throwing just a warning or nothing, returned value should be 0 or false, _script continues_