Improve the conversion method to do something reasonable with
empty Rectangle.
Use the conversion method in more places.

Change-Id: I48c13f3d6dae71f39f03f7939101e545c8125503
Reviewed-on: https://gerrit.libreoffice.org/71853
Tested-by: Jenkins
Reviewed-by: Regina Henschel <rb.henschel@t-online.de>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>