survive "TextBox" having no value

Change-Id: Iebed41cfcc5899387e3defb8d8a0792276349098
Reviewed-on: https://gerrit.libreoffice.org/13560
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>