PHP warning if there is no "whitelisted_domains" key present in the configuration

Problem/Motivation

Since extlink 8.x-1.2 ships with #3027186: Support simplified UI for multiple whitelisted domains it does not add an update function to set a new key "whitelisted_domains" visiting /admin/config/user-interface/extlink results in a warning:

Warning: implode(): Invalid arguments passed in Drupal\extlink\Form\ExtlinkAdminSettingsForm->buildForm() (line 193 of modules/contrib/extlink/src/Form/ExtlinkAdminSettingsForm.php).
Drupal\extlink\Form\ExtlinkAdminSettingsForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 520)
Drupal\Core\Form\FormBuilder->retrieveForm('extlink_admin_settings', Object) (Line: 277)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 694)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

Fix the configuration form if there is no "whitelisted_domains" key or write an update function.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet