Usecase to reproduce:
- Create a purchase requistion with type Call For Tender
- Update a line and set the price to zero.

UserError 'You cannot confirm the blanket order without price.' raised.
It happens because the write don't process the same check than create
and don't check if the purchase_requistion is a blanket order or a call
for tender. It also doens't check the current state of the
purchase_requisition.

closes odoo/odoo#39973

Task: 2120211
X-original-commit: 7d69f014
Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>