Add stub for \DOMNode::hasAttributes

[Niklan]

I'm not sure if the property should be explicitely declared. It works fine without it. Most likely, it comes from PHPStorm's stubs. Also, I'm not sure if it should be:

@phpstan-assert-if-true DOMNamedNodeMap $this->attributes

to be more specific, or is it fine with !null?

This PR fixes an issue where PHPStan does not understand chained calls, as discussed here: phpstan/phpstan#12495 (comment)

    if ($node->hasAttributes()) {
      foreach ($node->attributes->getIterator() as $attribute) {

  22     Cannot call method getIterator() on DOMNamedNodeMap|null.  
         🪪  method.nonObject