Apparently, I didn't have the latest Kendo version. There was a bug in 2016.1.112 that caused this. It appears to be fixed in 2016.1.226.
My other question remains unanswered however. How can I selectively "turn off" specific filter operators so that they don't show up in the UI? I have two reasons for wanting to do this. The first is that I don't have the time to write the code to implement the new operators in my server code at the moment. The second is that the "Is null" and "Is not null" operators are too geeky for the average end user. I could prefer to combine their functionality into the "Is empty" and "Is not empty" options when I do implement them.
One suggestion for you guys would be to interpret a null value setting for the operator text to mean that the operator should be removed. It doesn't do that currently, but it would be a straightforward way to give developers the option to selectively remove operators.