Fix inconsistent type-hint declaration

If 1 argument $validator in the constructor is null, it throws an error, which shouldn't be the case

Author: murtukov

src/Transformer/ArgumentsTransformer.php

@@ -27,10 +27,10 @@
 class ArgumentsTransformer
 {
     protected PropertyAccessor $accessor;
-    protected ValidatorInterface $validator;
+    protected ?ValidatorInterface $validator;
     protected array $classesMap;
 
-    public function __construct(ValidatorInterface $validator, array $classesMap = [])
+    public function __construct(ValidatorInterface $validator = null, array $classesMap = [])
     {
         $this->validator = $validator;
         $this->accessor = PropertyAccess::createPropertyAccessor();
@@ -67,7 +67,7 @@ private function getType(string $type, ResolveInfo $info): ?Type
     private function populateObject(Type $type, $data, bool $multiple, ResolveInfo $info)
     {
         if (null === $data) {
-            return $data;
+            return null;
         }
 
         if ($type instanceof NonNull) {
@@ -137,7 +137,7 @@ public function getInstanceAndValidate(string $argType, $data, ResolveInfo $info
 
         $result = $this->populateObject($this->getType($type, $info), $data, $isMultiple, $info);
 
-        if ($this->validator) {
+        if (null !== $this->validator) {
             $errors = new ConstraintViolationList();
             if (is_object($result)) {
                 $errors = $this->validator->validate($result);
@@ -152,7 +152,7 @@ public function getInstanceAndValidate(string $argType, $data, ResolveInfo $info
                 }
             }
 
-            if (count($errors) > 0) {
+            if ($errors->count() > 0) {
                 throw new InvalidArgumentError($argName, $errors);
             }
         }