$hfind() gives no return value when '$null' is passed. This indicates that it does not work, as it does not work with many identifiers.
We would have a problem if $hfind() returned 0 or some other value that isn't nothing. Nothing means Error.
Should, or can, it be made to work? Sure, why not. I could see it useful in cases of $hfind(table,$null,0).data to discover table entries with null empty values. Or, whatever the original poster is trying to do as well.