[FIX] core: unexpected display_name "False"

If a form view contains the field 'display_name', when creating a new
record, the initial value of 'display_name' is the string "False", and
that weird value also appears in the breadcrumb instead of "New".  In
order to avoid this unexpected behavior, the conversion of a value to a
display name should be False, like any other field would.

closes odoo/odoo#81788

Signed-off-by: Raphael Collet <rco@odoo.com>