FILESAVE: DOCX. Added null check to avoid crash.

Change-Id: I6f61d69702f77f0291ad25f5c39974d263d273e8
Reviewed-on: https://gerrit.libreoffice.org/7096
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>