Apply php-cs-fixer fix --rules nullable_type_declaration_for_default_null_value

Author: nicolas-grekas

PhpEngine.php

@@ -244,7 +244,7 @@ public function setHelpers(array $helpers)
  $this->addHelpers($helpers);
  }
 
- public function set(HelperInterface $helper, string $alias = null)
+ public function set(HelperInterface $helper, ?string $alias = null)
  {
  $this->helpers[$helper->getName()] = $helper;
  if (null !== $alias) {

TemplateReference.php

@@ -20,7 +20,7 @@ class TemplateReference implements TemplateReferenceInterface
 {
  protected $parameters;
 
- public function __construct(string $name = null, string $engine = null)
+ public function __construct(?string $name = null, ?string $engine = null)
  {
  $this->parameters = [
  'name' => $name,