[FIX] mail: controller avatar return placeholder if no content
[FIX] mail: controller avatar return placeholder if no content

binary_content return '' if no picture, so, we keep the placeholder
if no image has been returned.

It avoid in the new portal with discuss widget to get the default
browser 404 picture (file broken)

This commit close #12683

dfang/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo