[FIX] purchase: Update price unit on new line even if confirmed RFQ

Steps to reproduce:

  - Install purchase
  - Go to Settings and activate `Variant Grid Entry`
  - Create a new Requests for Quotation
  - Add a customer and add a product that has a variant min 2 variant
  - Wizard should ask for the variant
  - Select 1 variant by increasing quantity in the grid and confirm
  - Confirm order
  - Add again a product variant with the wizard

Issue:

  Price unit is not set on the new line.

Cause:

  In `_onchange_quantity` (triggered by the purchase_product_matrix
  module), we do not update price unit if order line
  is in state `purchase` or `done`.

Solution:

  Replace condition to not perform `_onchange_quantity` if order line
  has an invoice line.

opw-2956755

closes odoo/odoo#98960

Signed-off-by: Adrien Widart <awt@odoo.com>