fdo#72697 accept future <loext:sender-initials>

... of to-be <text:sender-initials>

Change-Id: Ia601c0debc5d66682aa92e14fb063395f9f48ab5
(cherry picked from commit 1379294741ff85d4c09d9ded7579ad50dea88fb7)
Reviewed-on: https://gerrit.libreoffice.org/7076
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>