Issue #3477178 by tr: Parameter typing in DataFilters

Closes #3477178

src/Plugin/TypedDataFilter/CountFilter.php

 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\typed_data\Plugin\TypedDataFilter;
 
 use Drupal\Core\Render\BubbleableMetadata;
@@ -41,7 +43,7 @@ class CountFilter extends DataFilterBase {
   /**
    * {@inheritdoc}
    */
-  public function filter(DataDefinitionInterface $definition, $value, array $arguments, BubbleableMetadata $bubbleable_metadata = NULL) {
+  public function filter(DataDefinitionInterface $definition, $value, array $arguments, ?BubbleableMetadata $bubbleable_metadata = NULL) {
     return mb_strlen($value);
   }