[FIX] account: prevent unnecessary recompute of invoice line account_id

Summary
------
Currently, when removing a product from an invoice line, the account for
that invoice line is automatically reset to a default value.

Steps to reproduce
------------------
* install `account_accountant`
* create an invoice and add a product
* set a different account on that invoice line
* remove the product from the line

You should see that the account is automatically changed.

opw-3092556

closes odoo/odoo#139361

X-original-commit: 39b44dbfef0d533722ce2c96d375ada113d32b00
Signed-off-by: de Wouters de Bouchout Jean-Benoît (jbw) <jbw@odoo.com>
Signed-off-by: Séna Serge Nshimiyimana (sesn) <sesn@odoo.com>

addons/account/models/account_move_line.py

@@ -1135,6 +1135,7 @@ class AccountMoveLine(models.Model):
     @api.onchange('product_id')
     def _inverse_product_id(self):
         self._conditional_add_to_compute('account_id', lambda line: (
+            (self.product_id or not self.account_id) and
             line.display_type == 'product' and line.move_id.is_invoice(True)
         ))
 

addons/account/tests/test_account_move_out_invoice.py

@@ -3771,3 +3771,26 @@ class TestAccountMoveOutInvoiceOnchanges(AccountTestInvoicingCommon):
             duplicate_invoice.line_ids.filtered(lambda l: l.display_type == 'payment_term').account_id,
             other_receivable_account
         )
+
+    def test_account_on_invoice_line_product_removal(self):
+        """Removing a product from an invoice line should preserve that line's account."""
+        other_income_account = self.product_a.property_account_income_id.copy()
+
+        invoice = self.env['account.move'].create({
+            'move_type': 'out_invoice',
+            'partner_id': self.partner_a.id,
+            'invoice_date': fields.Date.from_string('2019-01-01'),
+            'invoice_line_ids': [
+                Command.create({
+                    'product_id': self.product_a.id,
+                }),
+            ]
+        })
+        invoice.invoice_line_ids.account_id = other_income_account
+        invoice.invoice_line_ids.product_id = False
+
+        self.assertEqual(
+            invoice.invoice_line_ids.account_id,
+            other_income_account,
+            "Removing a product from an invoice line should no change the account."
+        )