This restores compatibility for API users who called getByName() on the
bitmap table and expected that the result can be set as a value for the
GraphicURL property.

The case is similar to the Writer images, which was handled in commit
587803ba (sw TextGraphicObject: handle
XBitmap for GraphicURL, 2018-08-08) already.

Change-Id: I3740a68989b306425d0d6c4d4e350ac163cb4170
Reviewed-on: https://gerrit.libreoffice.org/64069
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>