n#821567: Use BitmapURL only if its valid.

Ported from: bf2ff0468e06770f9320c652a2332f7ec137d061

Change-Id: I7464fcba352798a686fc0563dd023feac5234dbf
Reviewed-on: https://gerrit.libreoffice.org/8547
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>