[FIX] sale: prevent singleton error while creating invoices with sale credit

When the user set some sale credit, while creation of invoice of multiple sale
order which exceed the sale credit of a partner the user will get singleton
error.

steps to produce:-
- Install 'sale'
- Go to 'Setting' and enable the Sale Credit
- Create two sale order with same customer and product which has
  ordered quantities
- 'Conform' both the sale orders
- Select both the sale orders
- Go to actions and click 'Create Invoice'
- Traceback is generated

Applying this commit will fix this issue.

sentry-4109773100

closes odoo/odoo#120600

X-original-commit: 565d1ad4c679d47e391cea7badf60d85ac2aa9ca
Signed-off-by: John Laterre (jol) <jol@odoo.com>

addons/sale/models/account_move.py

@@ -150,17 +150,19 @@ class AccountMove(models.Model):
     def _compute_partner_credit(self):
         super()._compute_partner_credit()
         for move in self:
-            sale_order = move.line_ids.sale_line_ids.order_id
+            sale_orders = move.line_ids.sale_line_ids.order_id
             amount_total_currency = move.currency_id._convert(
                 move.tax_totals['amount_total'],
                 move.company_currency_id,
                 move.company_id,
                 move.date
             )
-            amount_to_invoice_currency = sale_order.currency_id._convert(
-                sale_order.amount_to_invoice,
-                move.company_currency_id,
-                move.company_id,
-                move.date
+            amount_to_invoice_currency = sum(
+                sale_order.currency_id._convert(
+                    sale_order.amount_to_invoice,
+                    move.company_currency_id,
+                    move.company_id,
+                    move.date
+                ) for sale_order in sale_orders
             )
             move.partner_credit += max(amount_total_currency - amount_to_invoice_currency, 0.0)