[FIX] account: no account in manual move creation

- Activate multi-currency
- Create an Journal Entry manually
- Create a line
  Do not fill Account
  Fill in Amount Currency
  Fill in Currency

A traceback occurs because `line.company_currency_id`.

An extra condition `and line.company_currency_id` is not sufficient to
have the onchange working properly. Indeed, the related field
`company_currency_id` goes through another related field (`company_id`),
leading to an empty value in some cases.

opw-1904854

closes odoo/odoo#28389

addons/account/models/account_move.py

@@ -701,7 +701,7 @@ class AccountMoveLine(models.Model):
             unaffected_earnings_type = self.env.ref("account.data_unaffected_earnings")
             record.is_unaffected_earnings_line = unaffected_earnings_type == record.account_id.user_type_id
 
-    @api.onchange('amount_currency', 'currency_id')
+    @api.onchange('amount_currency', 'currency_id', 'account_id')
     def _onchange_amount_currency(self):
         '''Recompute the debit/credit based on amount_currency/currency_id and date.
         However, date is a related field on account.move. Then, this onchange will not be triggered
@@ -709,9 +709,10 @@ class AccountMoveLine(models.Model):
         To fix this problem, see _onchange_date method on account.move.
         '''
         for line in self:
+            company_currency_id = line.account_id.company_id.currency_id
             amount = line.amount_currency
-            if line.currency_id and line.currency_id != line.company_currency_id:
-                amount = line.currency_id._convert(amount, line.company_currency_id, line.company_id, line.date or fields.Date.today())
+            if line.currency_id and company_currency_id and line.currency_id != company_currency_id:
+                amount = line.currency_id._convert(amount, company_currency_id, line.company_id, line.date or fields.Date.today())
                 line.debit = amount > 0 and amount or 0.0
                 line.credit = amount < 0 and -amount or 0.0