Improve readonly states for currency

Set readonly only if there is a currency set

invoice.py

@@ -258,7 +258,7 @@ class Invoice(OSV):
         required=True, states=_STATES, domain="[('party', '=', party)]")
     currency = fields.Many2One('currency.currency', 'Currency', required=True,
         states={
-            'readonly': "state != 'draft' or bool(lines)",
+            'readonly': "state != 'draft' or (bool(lines) and bool(currency))",
         })
     currency_digits = fields.Function('get_currency_digits', type='integer',
             string='Currency Digits', on_change_with=['currency'])