Replace usages of "Return success with data 'null'" with "EmptyResult"

[whimboo]

There a couple of references in the WebDriver BiDi spec that refer to Return success with data null, eg. the remote steps of the session.subscribe command. This is in conflict with the ResultData type that doesn't allow that, but defines the empty result as EmptyResult = {}.

As @jgraham said on Elements:

Returning null there is wrong, although maybe we could special case it in the caller. We should say:

1. Let |body| be a new map matching the EmptyResult production.
2. Return [=success=] with data |body|

The return null thing works in HTTP so I think that's where the error has come from.

[jgraham]

The other option is that we could special case null in the command processing algorithm, but that's probably more confusing for implementors.

[thiagowfx]

Does this need more discussion, or is it ready to go? I could create a PR, but I'd like to be sure there's no contentious point.