[FIX] crm: do not reset action name in convert wizard each time an option changes

Currently ``name`` field that holds the action type is recomputed each time
duplicated leads are updated. However duplicated leads are often updated
notably each time customer action (create / link / do nothing) and chosen
customer are updated.

As ``name`` compute method is mainly present to replace a default value
its compute method is updated so that it nows compute only when not having
a value. Once having a value user choice is kept.

Task ID-2452777
COM PR odoo/odoo#68618

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