Don't show sticky region if its empty

The sticky region also contains the page title, so I wonder when there is a scenario that this region might be empty. Can you explore on that use case a bit, please?

@juergenhaas You're right, this is a bit of an edge case. We expose a form on a specific url that can be embedded on third party websites and submitted by anonymous users. We do not have a title on that page.

Hello :),
Steps to test:

1. Configure the page title visibility to be hidden for the path: /admin/structure/block/list/gin
2. Navigate to /admin/structure/block/list/gin and verify that the div with class .region-sticky is not present in the DOM
3. Navigate to /node/add/article and confirm that the div with class .region-sticky is present in the DOM

This patch ensures that the sticky region is only rendered when it contains content.

checking

Testing this, I did get the empty space without the patch/MR applied. Applying the changes everything worked as expected.

The testing notes are useful for getting into the edge case of having an empty region. @anruether if you can describe what you do to come up with that region being empty then I can check further.

In any case, the MR is clean, well written and approved.

Well done all