Added a fallback to allow correct SvxShape construction
when SdrObject is not yet inserted - or has no SdrPage yet.
This is needed ue to the paradigm change that a SdrObject
only has a SdrPage when it is inserted.
For more info and a discussion, see added comments.

Change-Id: I2c1a4b1bbb531501ee73ab1c98c13321c5c0b050
Reviewed-on: https://gerrit.libreoffice.org/62710
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@cib.de>