[FIX] event_crm: fix lead/registration values propagation, improve tests

Fix various issues that appeared when working with registrations. When converting
registrations to leads using rules some convert flows have issues or can crash. This
PR fixes several issues, see sub commits for more explanations.

Improve test suite to cover more use cases.

Task-3431124

closes odoo/odoo#128823

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>