erp5_accounting: total assert price can be None

bt5/erp5_accounting/SkinTemplateItem/portal_skins/erp5_accounting/AccountingTransaction_roundDebitCredit.py

@@ -52,7 +52,7 @@ def roundLine(resource, get_method, set_method, exchange_ratio):
   precision = context.getQuantityPrecisionFromResource(resource)
   total_quantity = 0.0
   for line in line_list:
-    line_quantity = round(getattr(line, get_method)(), precision)
+    line_quantity = round(getattr(line, get_method)() or 0, precision)
     getattr(line, set_method)(line_quantity)
     total_quantity += line_quantity