fdo#81995: fix Outline numbering tab page

The "ParentNumbering" property is not a string.

(regression from 97eb8a6e0eb830f37dcba64a51d725aab4c5ff53)

Change-Id: Ib33e95847b388bb1126a63812e128c96a0c00730
(cherry picked from commit 4d3c9ed257e51af55f358d2b44d5fc2e341c7202)
Reviewed-on: https://gerrit.libreoffice.org/10875
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>