Declare $searchFilterPresent property

Fix warning :

Deprecated function : Creation of dynamic property Drupal\views_search_snippet\Plugin\views\field\Snippet::$searchFilterPresent is deprecated dans Drupal\views_search_snippet\Plugin\views\field\Snippet->query() (/home/clients/xxxxxxx/sites/xxxxx/web/modules/contrib/views_search_snippet/src/Plugin/views/field/Snippet.php ligne 30) 

Closes #3464679

src/Plugin/views/field/Snippet.php

@@ -17,6 +17,8 @@ use Drupal\views\ResultRow;
  */
 class Snippet extends FieldPluginBase {
 
+  private $searchFilterPresent = FALSE;
+
   /**
    * {@inheritdoc}
    */
@@ -45,7 +47,7 @@ class Snippet extends FieldPluginBase {
    */
   public function render(ResultRow $values) {
     // Only render if we exist.
-    if (isset($this->searchFilterPresent)) {
+    if ($this->searchFilterPresent) {
       $node_renderer = \Drupal::entityTypeManager()->getViewBuilder('node');
       $node = $this->getEntity($values);
       //Todo:langcode
@@ -65,4 +67,4 @@ class Snippet extends FieldPluginBase {
   }
 
 
-}
\ No newline at end of file
+}