fdo#81276: sw: HTML import: fix inverted test in CSS import

(regression from 5089bdb234a93bcb62b597c18b8ab66048246b13)

Change-Id: I5b45c4c47ba66385b2b528b5a3039bda37b060d6
(cherry picked from commit 7dff37c65e08985b80ac79e13f5571819a8e739d)
Reviewed-on: https://gerrit.libreoffice.org/10477
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>