Comment by mdorazio

Based on the description in the article, it's a separate system that is actually entering "NULL" in the license plate string field, probably for things like red light camera violations. Chances are it's a human doing this according to a procedure or the system is setup to require entry of some text in the field, so they have to enter something and opted for "NULL" if the actual plate is unreadable / unreliable / not present. This is unfortunately how a lot of things in the real world work, especially on legacy systems.