Issue #3506253 by joakland: Undefined array key 'watermark' on search

src/Plugin/Search/GoogleSearch.php

@@ -422,7 +422,7 @@ class GoogleSearch extends ConfigurableSearchPluginBase implements AccessibleInt
         ],
       ],
     ];
-    if (!$this->configuration['watermark']) {
+    if (!array_key_exists('watermark', $this->configuration)) {
       $output['#attached']['library'][] = 'google_cse/noWatermark';
     }
     if (!isset($this->configuration['results_accessibility']) || $this->configuration['results_accessibility'] !== FALSE) {