Port #12779 to v2int/2.0, since bohemia wants to integrate it

Send `null` instead of `undefined` in no-ops

There is a bug in 1.2 runtime where clients cannot handle ops whose
contents are `undefined`, since `JSON.stringify(undefined)` evaluates to
`undefined`, unlike `JSON.stringify(null)`, which evaluates to `"null"`.
Old clients try to access `length`, resulting in an error.