Issue #3410093 by bnjmnm: [regression] install_get_form does not allow...

Issue #3410093 by bnjmnm: [regression] install_get_form does not allow install_settings_form ['driver'] to be null

core/includes/install.core.inc

@@ -970,7 +970,7 @@ function install_get_form($form_id, array &$install_state) {
     $install_form_id = $form_builder->getFormId($form_id, $form_state);
     if (!empty($install_state['forms'][$install_form_id])) {
       $values = $install_state['forms'][$install_form_id];
-      if ($install_form_id === 'install_settings_form' && !str_contains($values['driver'], "\\")) {
+      if ($install_form_id === 'install_settings_form' && isset($values['driver']) && !str_contains($values['driver'], "\\")) {
         @trigger_error("Passing a database driver name '{$values['driver']}' to " . __FUNCTION__ . '() is deprecated in drupal:10.2.0 and is removed from drupal:11.0.0. Pass a database driver namespace instead. See https://www.drupal.org/node/3258175', E_USER_DEPRECATED);
         $driverExtension = Database::getDriverList()->getFromDriverName($values['driver']);
         $tmp = [];