[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 Comment Author: heiglandreas This is not a but. Similar illegal requests throw the same exception like for example new DateTime('null');. It is your responsibility as a developer to make sure that something sensible is passed to the constructor. As a side-note: UTC is not "forced". A timestamp is by it's very own [definition](https://en.wikipedia.org/wiki/Unix_time#Definition) "the seconds passed since 1st of January 00:00:00 UTC". So a timestamp is **always** based on UTC and a DateTime(Immutable) object created from one can not have any other timezone than UTC.