Your other option would have been to set up your formula to explicitly check for null, something like this:
IsNull({field}) or {field} <> 'T'
Nulls require special handling when doing comparisons - anything compared to Null is Null. So, for example, your {field} <> 'T' does NOT return false when {field} is null, it returns {null}. That's why you have to explicitly test for null and you need to do that check BEFORE you check for a value or it won't work correctly.
-Dell