PostAsync entity id check fails for Guid type

[jfhs]

Since Guid type does not have null value, it is always initialized to Guid.Empty and this check in JsonApiController fails (string value of Guid.Empty is "000-000...").

Looking around I was not able to find any generic way to check for empty values. So I would suggest adding method IsIdEmpty or something similar to IIdentifiable with default implementation maybe as you have it right now, or simply checking for null value. Then users could implement that method with whatever logic their ids have.

If that seems reasonable for you, I might make a pull request with that change tomorrow.