Fix workflow button display after form validation

Steps to reproduce :

1. define a workflow task with 2 buttons : one to validate (with no other condition) and one to reject with condition which requires to set a comment
2. override the _getValidity() function to test something else : for the purpose of the ticket, it returns always false
3. start the workflow and click to process the first task
4. observe that only the validate button is available ( the reject button is disabled)
5. click the validate button
6. observe the alert that the _getValidity() function returned false
7. observe that now the 2 buttons are active <<<=== that's the bug

Expected behavior: caling the _getValidity() function does not modify the conditions to display the workflow buttons