Compile render string without model

[vanillajonathan]

Is your feature request related to a problem? Please describe.
I would just like to use a view bag without any model.

Describe the solution you'd like
I want the model parameter in the CompileRenderStringAsync method to be optional.
So I can do:

var template = "Hello, @Model.Name. Welcome to RazorLight repository";
var viewBag = new ExpandoObject();
viewBag.TryAdd("Name", "John Doe");

string result = await engine.CompileRenderStringAsync("templateKey", template, null, viewBag);