Fix notices in the admin configuration form

Problem/Motivation

Warning: Invalid argument supplied for foreach() in Drupal\Core\Render\Element\Checkboxes::valueCallback() (line 100 of core/lib/Drupal/Core/Render/Element/Checkboxes.php).
Drupal\Core\Render\Element\Checkboxes::valueCallback(Array, , Object)
call_user_func_array(Array, Array) (Line: 1255)
Drupal\Core\Form\FormBuilder->handleInputElement('sharethis_settings', Array, Object) (Line: 972)
Drupal\Core\Form\FormBuilder->doBuildForm('sharethis_settings', Array, Object) (Line: 1042)
Drupal\Core\Form\FormBuilder->doBuildForm('sharethis_settings', Array, Object) (Line: 1042)
Drupal\Core\Form\FormBuilder->doBuildForm('sharethis_settings', Array, Object) (Line: 1042)
Drupal\Core\Form\FormBuilder->doBuildForm('sharethis_settings', Array, Object) (Line: 555)
Drupal\Core\Form\FormBuilder->processForm('sharethis_settings', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 74)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
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}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
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: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 652)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes