[FIX] contract: product is also required in contract line tree view

The Product is a mandatory field in the contract line form view

https://github.com/OCA/contract/blob/14.0/contract/views/abstract_contract_line.xml#L23

but it's not set as mandatory in the embedded tree view of the
contract line. This commit fixes this inconsistency.

This causes problems in particular with module contract_sale_generation,
since, unlike invoices, it's mandatory
to have a product on a Sales Order line.

contract/views/contract_line.xml

@@ -97,7 +97,11 @@
  decoration-info="create_invoice_visibility and not is_canceled"
  >
  <field name="sequence" widget="handle" />
- <field name="product_id" />
+ <field name="display_type" invisible="1" />
+ <field
+ name="product_id"
+ attrs="{'required': [('display_type', '=', False)]}"
+ />
  <field name="name" />
  <field
  name="analytic_account_id"