We need to export date format and also text content
in case of empty date field. Otherwise the exported
date field will be lost during import into LO Writer
or MSO Word.

Change-Id: I5cf65bedba010f64ca8f56262057f3cce32b0943
Reviewed-on: https://gerrit.libreoffice.org/66194
Tested-by: Jenkins
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>