[FIX] website_event: fix submenu without page

[jeh-odoo]

This commit odoo/odoo@af46b928a18da8e6276c107c5ed07cfd9e87d1bf introduced a fix to ensure submenu pages uniqueness.

An issue arises when manual submenu are created and don't have a view_id set. To avoid this issue, we now fallback on the initial page receives if no view is found.

[robodoo]

[awa-odoo]

@robodoo delegate=@jeh-odoo

[jeh-odoo]

@robodoo r+