Fix type error when response is missing included

[pelargir]

This fixes the following error when the response from the server does not have the included key:

TypeError: Cannot read properties of undefined (reading 'included')

When missing, the value already defaults to an empty array so this should be safe when it's "undefined" as well.

[tvdeyen]

If I read the error message correctly the response object is undefined and this does not have the included key - what's expected, because the response is null/undefined.

I don't think it's the responsibility of the deserialization code to check if the response is null/undefined. This should be handled in the response handler of your Ajax/fetch call.

[pelargir]

You're right. This happens when the response is null. It doesn't have anything to do with the included key.

I think good defensive code that gets called from outside the library should absolutely check for the presence of a response before acting upon it, but I understand why you might disagree.