[FIX] web,sale_product_matrix: allow to open matrix

Before this rev, it crashed when opening the (sale or purchase)
product matrix (e.g. on runbot, go to quotations, create, add a
line, select 'My Company tshirt').

This was due to a recent override of _applyChanges (see 4bf98b47),
which didn't return the value returned by super.

OPW 2116083

closes odoo/odoo#39717

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>