prepare to read loext:vertical-justify, fdo#72922

Change-Id: I56d72d5cb8ea95aa63d4659899cba2f295ce9bea
(cherry picked from commit 11a73705d9dd10ebefa2bcdefa560f55e7024a1b)
Reviewed-on: https://gerrit.libreoffice.org/7150
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>