Steps:
- Login as an admin user
- On a fresh branch install event_crm
- Give a "test" user access rights of "Own Document only" in the "Sales"
  app, nothing in "Events"
- Assign any lead to "test" user
- Login as "test" user and try to access your own leads

Bug:
Error: While parsing modifiers for button: for modifier "invisible":
Unknown field registration_count in domain

Explanation:
The button `event_registration_action_from_lead` tries to access event
registrations even if the user doesn't have access to events.

opw:2462127

closes odoo/odoo#66425

Signed-off-by: backspac <backspac@users.noreply.github.com>