serve-d is too strict in LSP enforcement (crashes)

[gdamore]

(The other side of this was reported at https://github.com/zed-industries/zed/issues/11060 )

Serve-d crashes when it gets:

{"jsonrpc":"2.0","id":1,"result":null,"error":null}

Essentially according to Postel's Law, we should be a bit more tolerant of reasonable errors.

Clearly the above message can reasonably be interpreted as intended to be:

{"jsonrpc":"2.0","id":1,"result":null}

(Having a null value for "error" is nonsense per the spec, but having a null value is perfectly reasonable as a result from function that has nothing else to return back.)