adds null coalescing operator check for bootstrap key index (#4685)

Fixes issue for inspector check for installed Drupal bootstrap value when not installed and no value is returned.

Co-authored-by: Dane Powell <git@danepowell.com>

src/Robo/Inspector/Inspector.php

@@ -189,7 +189,7 @@ public function isDrupalSettingsFileValid() {
    */
   public function isDrupalInstalled() {
     $this->logger->debug("Verifying that Drupal is installed...");
-    $output = $this->getDrushStatus()['bootstrap'];
+    $output = $this->getDrushStatus()['bootstrap'] ?? '';
     $installed = $output === 'Successful';
     $this->logger->debug("Drupal bootstrap results: $output");