[FIX] product,web: color_field fixes for product variant color

There was a typo in the color_field xml not allowing to disable the
pop-up in case of readonly. A test validates the feature is active.

The color_field now displays the transparent symbol (instead of black
color) in case the color is undefined.

The product configurator also displays the transparent symbol for
undefined non-custom colors.

task-3013110

closes odoo/odoo#103478

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>