Steps to reproduce:

- Install Events
- Activate debug mode
- Events > Config > Lead generation > Create

Traceback is raised.

The `debugValue` prop added in a29691bf should be a string but doesn't
have a default value, and it is evaluated as `false` at runtime.

Solution:

Add default value for `debugValue`.

opw-3056391

closes odoo/odoo#109238

Signed-off-by: Michaël Mattiello <mcm@odoo.com>