code: private function getFieldArgNode($type, $fieldName, $argName)
      {
          $nodes = $this->getAllFieldArgNodes($type, $fieldName, $argName);
  
          return $nodes[0] ?? null;
      }
docs: @param ObjectType|InterfaceType $type
  @param string                   $fieldName
  @param string                   $argName
  
  @return InputValueDefinitionNode|null