Caption fails to render when adding back to captionless embed when edited via EntityEmbedDialog

I have test coverage demonstrating this bug in this patch:

https://www.drupal.org/files/issues/2019-06-09/entity-embed-3060396-11.p...

    // @todo remove the back and forward.
    // See https://www.drupal.org/project/entity_embed/issues/3060439
    $this->assertSession()->elementNotExists('css', 'figcaption');
    $this->pressEditorButton('undo');
    $this->pressEditorButton('redo');

    $this->getSession()->switchToIFrame('ckeditor');
    $figcaption = $this->assertSession()
      ->waitForElementVisible('css', 'figcaption');

I suspect the cause is this:

Changes made to the data fire the data event.

From https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_plugins_widget.h...

I suspect it's that when you delete the property and add it back, it doesn't detect that a property has changed. Not confirmed, just a hunch. But this seems to fix it:

               if (values.attributes['data-caption']) {
                 values.attributes['data-caption'] = CKEDITOR.tools.htmlDecodeAttr(values.attributes['data-caption']);
               }
+              else {
+                values.attributes['data-caption'] = '';
+              }

My reasoning is that by keep the attribute set to a blank string, when it's updated, it detects that it has changed.

I didn't want to write a separate test for this issue, so I'm posting an updated patch to #3060396: Add CKEditor Widget JS test coverage.

The test only patch should fail and demonstrate the bug.

We might want to just reinitialize the widget. That would probably also fix #3060634: Editing link text in captions via EntityEmbedDialog fails to update link text in CKEditor Widget

Although I'm sure there are advantages with undo/redo not to reinitialize unnecessarily.

Can we just reinitialize the editable caption, when it changes?

We could also put this in as a stop-gap, since it doesn't seem to hurt anything, until we find a fix that encompasses both bugs (and perhaps others).