[BUGFIX] Fix php warning undefined array key no_search_sub_entries

To avoid php warning for undefined array key, check if key is set in
$page array.

Fixes TYPO3-Solr#3380

Classes/IndexQueue/RecordMonitor.php

@@ -200,7 +200,7 @@ protected function skipRecordByRootlineConfiguration(int $pid): bool
             return true;
         }
         foreach ($rootline as $page) {
-            if ($page['no_search_sub_entries']) {
+            if (isset($page['no_search_sub_entries']) && $page['no_search_sub_entries']) {
                 return true;
             }
         }