Fix 8.4 deprecation's

Author: Andrew Scott

src/NodeList.php

@@ -30,7 +30,7 @@ class NodeList extends NodeCollection
  * @param Document $document
  * @param iterable $nodes
  */
- public function __construct(Document $document = null, ?iterable $nodes = null) {
+ public function __construct(?Document $document = null, ?iterable $nodes = null) {
  parent::__construct($nodes);
 
  $this->document = $document;

src/Traits/TraversalTrait.php

@@ -297,7 +297,7 @@ public function eq(int $index): ?\DOMNode {
  *
  * @return NodeList
  */
- public function parents(string $selector = null): NodeList {
+ public function parents(?string $selector = null): NodeList {
  return $this->parentsUntil(null, $selector);
  }