Is your question really about why a blank or null date satisfies the match criteria?
Try adding a calc filed to the BAQ that is set by if the date filed is null. Make it a char field for something like (pseudo-code):
calc_value = “YB”;
calc_value = “NB”;
Then in the Dashboard’s tracker, add a field for “Blank Date”, to MATCH the calc field you added.
“Y” to see only results with blank dates
“N” to see only results with non-blank dates (and in the range of your other fields
“B” to see both
If you don’t ever need to see both at the same time. Just make the calc field a boolean