fdo#74155 mailmerge output path fix

regression from dd7acb3d76cd89b3f05dbdafe88eb50e60fd9d10

Change-Id: Ia79ed67df1f0e2803bd3e729951dee3a7438096d
Reviewed-on: https://gerrit.libreoffice.org/8729
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>