[FIX] web_editor: allow to simulate a preview of a colorpicker reset

This is important so that trigger-value works
(see https://github.com/odoo/odoo/commit/e590609ebb429c6254766aa1fd8cab3ce3fd4c74

)

closes odoo/odoo#43182

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>