[FIX] mass_mailing: prevent traceback in _getCSSColorValue

mass_mailing has an override of _getCSSColorValue which was basically
never used since the option it's linked to was mostly inactive. Now it's
active it became obvious there is a mistake in it that causes a
traceback to occur when no color is passed to it. This adds the same
check as in the original method and thereby prevents the traceback.

task-2760157

closes odoo/odoo#84926

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/static/src/js/mass_mailing_snippets.js

@@ -205,6 +205,9 @@ options.registry.ImageTools.include({
      * @override
      */
     _getCSSColorValue(color) {
+        if (!color || ColorpickerWidget.isCSSColor(color)) {
+            return color;
+        }
         const doc = this.options.document;
         if (doc && doc.querySelector('.o_mass_mailing_iframe') && !ColorpickerWidget.isCSSColor(color)) {
             const tempEl = doc.body.appendChild(doc.createElement('div'));