[FIX] mail: only set module icon if possible
[FIX] mail: only set module icon if possible

Posting a message on a custom model (i.e. created with Studio) crashed.

If it's a custom model, its `_original_module` attribute is not correctly set so
trying to retrieve the icon based on this attribute does not work.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo