No default url on countdown's urlpicker anymore. Only a placeholder.
Also redirect to homepage if no url.

Part of: https://github.com/odoo/odoo/pull/46640



task-2210358

closes odoo/odoo#47235

X-original-commit: a0d983e94e3681a3f3e55cc445defc1ec862c606
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>