[BUGFIX] NPE in Index Queue module when no site is selected

In multi site setups a null pointer exception is thrown when opening the
Index Queue TYPO3 module without having a solr-enabled page selected in
the page tree. The NPE is thrown in `::initializeAction()` before the
original handling of the "no site selected" case in `::indexAction()`.

Fixes: 3ea2713 ("[TASK:BP:11.6] Consider queue initialization status")

Classes/Controller/Backend/Search/IndexQueueModuleController.php

@@ -336,6 +336,10 @@ protected function addIndexQueueFlashMessage(string $label, int $severity)
      */
     protected function getIndexQueues(): array
     {
+        if ($this->selectedSite === null) {
+            return [];
+        }
+
         $queues = [];
         $configuration = $this->selectedSite->getSolrConfiguration();
         foreach ($configuration->getEnabledIndexQueueConfigurationNames() as $indexingConfiguration) {