As for validator required/requiredif , the continuous space string (ex. " ") is jugded blank field.
But 'null' of validwhen shows only null or length 0 String .
For ex,'this == null' is not hit the continuous space string (ex. " ").
I think it should be same behavior because 'Validator Guide' described as follows:
> requiredif - field dependant validator
>Deprecated, use validwhen.
Now, requiredif cannot be replaced validwhen.