Fix phpstan errors in php 8.4

Author: Marcin Czarnecki

src/AbstractVisitor.php

@@ -14,7 +14,7 @@
 abstract class AbstractVisitor implements VisitorInterface
 {
     /**
-     * @var GraphNavigatorInterface
+     * @var GraphNavigatorInterface|null
      */
     protected $navigator;
 

src/JsonDeserializationVisitor.php

@@ -204,7 +204,7 @@ public function endVisitingObject(ClassMetadata $metadata, $data, array $type):
      */
     public function getResult($data)
     {
-        unset($this->navigator);
+        $this->navigator = null;
 
         return $data;
     }

src/JsonSerializationVisitor.php

@@ -190,7 +190,7 @@ public function setData(string $key, $value): void
      */
     public function getResult($data)
     {
-        unset($this->navigator);
+        $this->navigator = null;
 
         $result = @json_encode($data, $this->options);
 

src/XmlDeserializationVisitor.php

@@ -449,7 +449,7 @@ public function revertCurrentMetadata()
      */
     public function getResult($data)
     {
-        unset($this->navigator);
+        $this->navigator = null;
 
         return $data;
     }

src/XmlSerializationVisitor.php

@@ -383,7 +383,7 @@ public function endVisitingObject(ClassMetadata $metadata, object $data, array $
      */
     public function getResult($node)
     {
-        unset($this->navigator);
+        $this->navigator = null;
         if (null === $this->document->documentElement) {
             if ($node instanceof \DOMElement) {
                 $this->document->appendChild($node);