Do not compute sequence next number if the SQL sequence does not exist

(grafted from c4554eeb56fd98c67b2fbade54dd0f5816d44c79)

--HG--
branch : 6.0

trytond/trytond/ir/sequence.py

@@ -166,8 +166,10 @@ class Sequence(DeactivableMixin, ModelSQL, ModelView):
 
         transaction = Transaction()
         if sql_sequence and not self._strict:
-            return transaction.database.sequence_next_number(
-                transaction.connection, self._sql_sequence_name)
+            if transaction.database.sequence_exist(
+                    transaction.connection, self._sql_sequence_name):
+                return transaction.database.sequence_next_number(
+                    transaction.connection, self._sql_sequence_name)
         else:
             return self.number_next_internal