Set empty currency when reconcile no lines

Since f9560247a0fc the currency for the reconciled lines is computed but when
there is no line, there is no company to get currency from.

Closes #13047

modules/account/move.py

@@ -2003,7 +2003,7 @@ class ReconcileLines(Wizard):
 
     def get_writeoff(self):
         "Return writeoff amount and company"
-        company = None
+        company = currency = None
         amount = Decimal('0.0')
         amount_second_currency = Decimal('0.0')
         second_currencies = set()
@@ -2014,6 +2014,7 @@ class ReconcileLines(Wizard):
             second_currencies.add(line.second_currency)
             if not company:
                 company = line.account.company
+                currency = company.currency
         try:
             second_currency, = second_currencies
         except ValueError:
@@ -2021,8 +2022,6 @@ class ReconcileLines(Wizard):
         if second_currency:
             amount = amount_second_currency
             currency = second_currency
-        else:
-            currency = company.currency
         return amount, currency, company
 
     def transition_start(self):