...that had been introduced with 5187d3ae
"Resolves: tdf#69042 - Add a 'What's New' infobar":

* Make the default value nil instead of an empty string.

* A nilable prop can't reuse the legacy utl::ConfigManager framework to obtain
  string prop values, so drop it from there and directly use the direct-access
  functionality in SfxViewFrame::Notify.

Change-Id: I5fc67a3d7e0fcb44a218889ecf8c69e81661567e
Reviewed-on: https://gerrit.libreoffice.org/72485
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>