fix odf validation errors around new chart regression curves

Change-Id: If67207abb204b5aca8365737088437b12ac6eaf1
Reviewed-on: https://gerrit.libreoffice.org/8523
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>