Missing data not being treated as nulls in alert filter

I’m reading from a MySQL database and have a single query to demonstrate my issue.

The query measures some disk usage, and I wanted to set an alert up for when that usage has gone over a certain threshold. I also want the absense of the usage metric to be treated as null and trigger an alert as well. However, I noticed that when the usage metric is not pushed, even though I have set no data to be treated as nulls and set to “Alerting” state, the alert still does not get triggered.

Has anyone else had this issue or know what I’m doing wrong? Here’s a screenshot of my alert page for the panel as an example: