It is possible that we not only have a dummy OLE object, but both the
replacement image and the native data is missing. Handle these enough so
that we don't give up exporting the document and also produce valid
output.

Change-Id: Ia973ecfbfb7e8fdecdc831b29d3a82c988ed7ea5
Reviewed-on: https://gerrit.libreoffice.org/59806Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins