[FIX] account: liquidity-line currency journal ok

If we do a payment with:
- a currency different than the journal
- the journal currency the same as company currency

=> it fails, because of the constraint on account.move.line:

> The amount expressed in the secondary currency must be positive when
> account is debited and negative when account is credited.

with code:

```
CHECK(currency_id IS NULL OR company_currency_id IS NULL OR
((currency_id != company_currency_id) AND {conditions on balance}))
```

This happens because in this use case, a NON-NULL currency_id means that
the currency is different than the company currency, this was not taken
into account when creating a liquidity line.

opw-2150267
closes #41244

closes odoo/odoo#41278

X-original-commit: f9d75e3a5442162538c727eb7e4f111c9b792104
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/account/models/account_payment.py

@@ -482,7 +482,11 @@ class account_payment(models.Model):
             # Manage custom currency on journal for liquidity line.
             if payment.journal_id.currency_id and payment.currency_id != payment.journal_id.currency_id:
                 # Custom currency on journal.
-                liquidity_line_currency_id = payment.journal_id.currency_id.id
+                if payment.journal_id.currency_id == company_currency:
+                    # Single-currency
+                    liquidity_line_currency_id = False
+                else:
+                    liquidity_line_currency_id = payment.journal_id.currency_id.id
                 liquidity_amount = company_currency._convert(
                     balance, payment.journal_id.currency_id, payment.company_id, payment.payment_date)
             else: