Pages: `get_page_uri()` should return the URI at which the resource being...

Pages: `get_page_uri()` should return the URI at which the resource being accessed is available at, this may include non-'publish' status posts.

Reverts [34001] and fixes the original issue in #15963 - avoiding a PHP Notice for when the post doesn't exist.

Props tharsheblows.
See #15963.
Fixes #35084.

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