This commit makes a small adjustment to the course view to hide the responsible
label in the course description if none is set.

As this field is not required on the slide.channel model, it's not necessary to
show a useless label when it's not set.

Task 2368573

closes odoo/odoo#60579

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>