FDO#76013 : If original file has image whose link is broken RT gets corrupted.

- The original file contains an image which has invalid link.
- since the link is invalid the import throws an exception while loading
  the image & the link is lost.
- while exporting the system used to write an empty image for an unhandled
  graphic type, which is incorrect and the RT file would result in corrupt.
- Fixed this issue and added UT.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/8549

Change-Id: If884f154d1fba921280d63930e7fccb979ea5608