Otherwise LO crashes e.g. at opening Expert Configuration dialog.
Instead, log the event, create an empty backend, and return empty values.

Change-Id: I433fc89c003e7886dfc5242a4ef8defa46d643d3
Reviewed-on: https://gerrit.libreoffice.org/53958Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>