Resolve #3461936: Fix str_replace() called with NULL

Closes #3461936

src/Plugin/search_api/processor/Highlight.php

@@ -471,6 +471,11 @@ class Highlight extends ProcessorPluginBase implements PluginFormInterface {
   protected function createExcerpt($text, array $keys) {
     // Remove HTML tags <script> and <style> with all of their contents.
     $text = preg_replace('#<(style|script).*?>.*?</\1>#is', ' ', $text);
+    // In case the preg_replace() failed for some reason it's safer to just
+    // return NULL instead of risking returning Javascript as th excerpt.
+    if ($text === NULL) {
+      return NULL;
+    }
 
     // Prepare text by stripping HTML tags and decoding HTML entities.
     $text = strip_tags(str_replace(['<', '>'], [' <', '> '], $text));