SdrText::SetOutlinerParaObject was modified to not check for
self-assign, and instead assert because
the existing check was no longer possible.

Fix bug in SdrUndoObjSetText::Undo(), where it was calling
SdrText::SetOutlinerParaObject unnecessarily,
because NbcSetOutlinerParaObjectForText already does that.

Optimise Outliner::GetEmptyParaObject by creating a new constructor for
OutlinerParaObject,
so we don't need to copy the new object we get back from
GetEmptyTextObject, unnecessarily.

Change-Id: I57c475583d6c31658c154e24992b3d587bad9841
Reviewed-on: https://gerrit.libreoffice.org/56730
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>