If this is an entity attribute of type string, you can check the following:
$EntityName/Attribute = empty this checks to see if a string is null
$EntityName/Attribute = ‘’ this checks to see if it is an empty string
Hope that helps,
You could also use the community commons IsEmptyString or IsNotEmptyString actions. Sometimes easier then doing the above checks.
As Mike suggested, you can use the following condition to validate the String whether it is empty or null.
To check both at a time in a single exclusive split, use the same condition like below
$EntityName/Attribute = empty or $EntityName/Attribute = ‘’