show blank page when no image is found and dialog aborted

[bertsky]

Currently, when all of the facilities to find /PcGts/Page/@imageFilename on disk fail (CWD, resolve-dir, or user dialog), PageViewer cancels everything, showing no document (as if called without arguments).

It would be nice though if instead it would at least show all annotations on a blank page (as if disabling the Image button), so they can still be inspected.

Use-case: the actual image is gone or hard to find, but viewing annotations is better than nothing.