Workfront only pulls on data present - so this isn't natively possible.
A workaround you could use is to crate a task form that is visible only to admins with a calculated field on it like Tasks Counted, default the number to 1, and apply it to your existing tasks and templated tasks.
From that perspective you'd be able to report on all tasks assigned without hours tracked.
You could likely pull a report also equal to actual hours is blank but....whicher works for you!
