Apply fixes from StyleCI

src/Pug/AbstractFilter.php

@@ -9,7 +9,7 @@
  */
 abstract class AbstractFilter extends AbstractFilterBase implements FilterInterface
 {
-    public function __invoke($code, array $options = null)
+    public function __invoke($code, ?array $options = null)
     {
         return $this->pugInvoke($code, $options);
     }

src/Pug/Filter/AbstractFilterBase.php

@@ -9,7 +9,7 @@ abstract class AbstractFilterBase
 {
     use WrapTagTrait;
 
-    public function pugInvoke($code, array $options = null)
+    public function pugInvoke($code, ?array $options = null)
     {
         if (method_exists($this, 'parse')) {
             $code = $this->parse($code, $options);

src/Pug/Filter/Php.php

@@ -9,7 +9,7 @@
  */
 class Php implements FilterBase
 {
-    public function __invoke($code, array $options = null)
+    public function __invoke($code, ?array $options = null)
     {
         return "<?php\n$code\n?>";
     }

src/Pug/FilterInterface.php

@@ -13,5 +13,5 @@ interface FilterInterface
      *
      * @return string
      */
-    public function __invoke($code, array $options = null);
+    public function __invoke($code, ?array $options = null);
 }