Reset HiddenField if AutoCompleteExtender returns no results

Question

Hi i need to be able to reset a HiddenField's value if my AutoCompleteExtender returns no results (specifically, if the user searches for something that's not in the list/database). I have this JS code now:

function autoCompleteItemSelected(source, eventArgs) {
    var assocHiddenField = document.getElementById(source.get_id() + '_hidden');
    assocHiddenField.value = eventArgs.get_value();
}

How would I modify this to check if the list is null? Currently it seems to just leave the value as it was previously.

Thanks