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>
Reviewed-by: Bjoern Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit 6181b3fe2c11d55f46eaaf657877aa9396c16374)