[FIX] website_event: unblock button when no reserve

The website_event_questions override EventRegistrationForm's method
on_click and expects the returns is a deferred.

This commit provides a deferred in all instances avoiding an error.

Also what was done in e9454e79 and enables back the Register Now
button if no ticket was selected.

opw-1850527

In 11.0, this change e9454e79 solved the use case of:

- opening the registration of a ticket
- discard

=> the page must be reloaded to register a ticket

A new report is that since 9.0, if we try to register 0 ticket we would
also have to reload the page.

This commit backports e9454e79 and solves the 0 ticket registration.
Also if website_event_questions was installed there was an error
preventing an error message to be displayed.

saas-15 version of 9.0's #24966
opw-1851622
closes #24967