Add DowngradeDomNodeChildNodesForeachRector for null $childNodes before PHP 8.0

[TomasVotruba]

Why

As of PHP 8.0, DOMNode::$childNodes always returns a DOMNodeList (php-src#5180, bug #79271). On PHP < 8.0 it returns null for nodes that cannot have children (e.g. DOMText, DOMDocumentType).

So code written for 8.0+ that iterates childNodes directly:

foreach ($node->childNodes as $childNode) { ... }

emits Warning: Invalid argument supplied for foreach() once downgraded and run on PHP 7.x. This was hit in real downgraded code (e.g. nunomaduro/termwind's Node::getChildNodes()).

What

New DowngradeDomNodeChildNodesForeachRector (PHP 8.0 set) that null-coalesces a foreach over DOMNode::$childNodes:

-foreach ($node->childNodes as $childNode) {
+foreach ($node->childNodes ?? [] as $childNode) {

It only fires when:

• the iterated expression is a childNodes property fetch, and
• the caller's type is DOMNode or a subtype (DOMElement, etc.).

Scoped to foreach on purpose: that is the documented warning and ?? [] is unambiguously safe there. Already-coalesced expressions, other properties (attributes), and non-DOM objects are skipped (covered by fixtures).

Registered in config/set/downgrade-php80.php. ECS, PHPStan and PHPUnit pass.

[TomasVotruba]

Fixes bugs in tomasvotruba/lines using termwind that uses XML 👍