link type field Support

To reproduce:

1. create a link type field, set it to internal only, not required.
2. create a link type field, set it to external only, not required.
3. create a list (integer) field, required with the following options:

1|Internal
2|External

4. Add field states on the two link fields, visible based on the value of the list field.

If you leave the link fields empty and save the node, you'll see the following notice repeated:

Notice: Undefined index: uri in Drupal\link\Plugin\Field\FieldWidget\LinkWidget->massageFormValues() (line 335 of core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php).
Drupal\link\Plugin\Field\FieldWidget\LinkWidget->massageFormValues(Array, Array, Object) (Line: 376)
Drupal\Core\Field\WidgetBase->extractFormValues(Object, Array, Object) (Line: 222)
Drupal\Core\Entity\Entity\EntityFormDisplay->extractFormValues(Object, Array, Object) (Line: 326)
Drupal\Core\Entity\ContentEntityForm->copyFormValuesToEntity(Object, Array, Object) (Line: 293)
Drupal\Core\Entity\EntityForm->buildEntity(Array, Object) (Line: 147)
Drupal\Core\Entity\ContentEntityForm->buildEntity(Array, Object) (Line: 278)
Drupal\Core\Entity\EntityForm->submitForm(Array, Object) (Line: 137)
Drupal\Core\Entity\ContentEntityForm->submitForm(Array, Object)
call_user_func_array(Array, Array) (Line: 111)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 585)
Drupal\Core\Form\FormBuilder->processForm('node_page_edit_form', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm('node_page_edit_form', 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: 582)
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: 153)
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: 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: 657)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Removing the field states ui configuration on the two link fields suppresses the notices.

I'll see if I can come up with a patch for this.