iAPI Router: don't remove region when it's not present in the new page

[luisherranz]

Description

Right now, if a page has a region and you navigate to a page that doesn't contain that region, the region gets deleted. We should consider whether this is the expected outcome or if it should instead maintain the regions intact if these regions are not found on the new page.

Step-by-step reproduction instructions

1. Load a page that has a region using the data-wp-route-region directive and a block that navigates to a URL that doesn't have that region.
2. Start the navigation.
3. The region disappears.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes