Picking a shape for an image in mass_mailing (when in developer mode)
would result in a sudden error due to a None
being passed to _getCSSColorValue

The shape picker is completely hidden in  the mass_mailing
templates editor as essentially no mail
client supports it in this version up to 15.3

Feature properly reintroduced in:
https://github.com/odoo/odoo/pull/84926



Task-2937490

closes odoo/odoo#97042

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>