Do not fail on missing attribute in InvoiceTax._key

issue11136
review370911002
(grafted from ac674a252a05d63c3ee61a277d7abeb394edc10f)

--HG--
branch : 6.0

invoice.py

@@ -2587,8 +2587,10 @@ class InvoiceTax(sequence_ordered(), ModelSQL, ModelView):
     @property
     def _key(self):
         # Same as _TaxKey
-        tax_id = self.tax.id if self.tax else -1
-        return (self.account.id, tax_id, self.base >= 0)
+        tax_id = self.tax.id if getattr(self, 'tax', None) else -1
+        account_id = (
+            self.account.id if getattr(self, 'account', None) else None)
+        return (account_id, tax_id, (getattr(self, 'base', 0) or 0) >= 0)
 
     @classmethod
     def check_modify(cls, taxes):