Use list comprehension in Invoice._credit().

review644002
(transplanted from 321da7e76acd53dc728631d1094010c36acd7de0)

invoice.py

@@ -1797,8 +1797,8 @@ class InvoiceLine(ModelSQL, ModelView):
             res[field] = getattr(getattr(self, field), 'id', None)
 
         res['taxes'] = []
-        for tax in self.taxes:
-            res['taxes'].append(('add', tax.id))
+        if self.taxes:
+            res['taxes'].append(('add', [tax.id for tax in self.taxes]))
         return res