fdo#74325: Ensure that all note objects have an sdr object before exporting.

Otherwise, if a note hasn't been displayed at least once before saving
that note would not get saved.  In the future, we should modify the export
code to not rely on SdrObject to check the presence of note.

Change-Id: Ib7ca3ac00a0c9cdd3a01facda7af479ef172afbe
(cherry picked from commit 018500a73f3b1082b6662b7c123dfe5158ae5752)
Reviewed-on: https://gerrit.libreoffice.org/7782
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>