It looks like these values are not nulls - they are missing values. Missing values are never evaluated, so it does not matter what expression you use. NullasValue also will not help you - this is a data model design issue, and you will need to provide details of your data sources, expressions and load scripts (and a small sample qvf).
And for the record, Alt() is not intended to catch nulls. Rather it returns the first valid numeric value in its list of arguments. So it filters non-numerics (eg invalid dates) and cannot be used to filter nulls for a text field.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein