`Response.Result` of null should still be serialized

[ryanbrandenburg]

By my reading of the LSP spec the result property is required in all successful cases, but currently if a null response if given for, for example, textDocument/hover the response will not contain the result property. I did some digging and am pretty sure this can be fixed by changing this area to unconditionally return a result property, setting it to null if it's null.

I'll try to send a PR with my proposed fix.