[IMP] purchase: do not make the field required
[IMP] purchase: do not make the field required

The value is made by the computed field.
Setting one has no effect (no inverse method).

If you have no value on a purchase order, you end up in the confusing situation
where you can not save a new PO without setting a value to date_planned but,
after saving, the value is reset (no lines).

Do not block the creation of a new purchase order without lines

Fixes #23275

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo