Add null checks for JSON stream

[Seelenoede]

In a project, I tried to create a mock Order record with a few OrderItems. But then makeRelationship failed.
After further digging, I noticed that one of the Product2 had a field with a value, while another Product2 had this field empty, which then lead to an issue with the "streamTokens method in fflib_ApexMocksUtils, as it couldn't parse the null token.
So basically it tried to write the JSON as
{ "data": "xyz",
"data2":,
"data3: "abc"}
which is of course is an invalid JSON.

I could fix it by adding

else if(currentToken == JSONToken.VALUE_NULL)
	toStream.writeNull();

to the method.

I also saw that there is no parsing for NOT_AVAILABLE and VALUE_EMBEDDED_OBJECT, although I don't think, that is an issue.

It would be great to have the fix from above part of the code, unless there is a good reason why we shouldn't parse null values.

[ImJohnMDaniel]

@Seelenoede -- Thanks for letting us know about this issue. Any chance that you could create some examples demonstrating the behavior, please? It would help us resolve this quicker. I appreciate it.

[Seelenoede]

@ImJohnMDaniel sure. I could reproduce this on a Trailhead Playground with the current version of apex-mocks. A set up an unit test.
What I am doing is, I create at least two Product2 records. Both have the same fields filled, but one field is set to null.
Then I create an OrderItem record for each of the products and finally add my order items to an Order with fflib_ApexMocksUtils.makeRelationship().
If I run the test, makeRelationship will throw the JSONException as it will not parse the null value of the Product2 correctly.

@IsTest
public with sharing class fflibUnitTest {
    @IsTest
    static void shouldNotThrowErrorWhenMockingRelationship() {
        try {
            Product2 prod1 = new Product2(
                Id = fflib_IDGenerator.generate(Product2.SObjectType),
                Name = 'Product1',
                ProductCode = 'P1',
                Description = null,
                StockKeepingUnit = 'P1'
            );

            Product2 prod2 = new Product2(
                Id = fflib_IDGenerator.generate(Product2.SObjectType),
                Name = 'Product2',
                ProductCode = 'P2',
                Description = 'this is another product',
                StockKeepingUnit = 'P2'
            );

            OrderItem oi1 = new OrderItem(
                Id = fflib_IDGenerator.generate(OrderItem.SObjectType),
                Product2Id = prod1.Id,
                Product2 = prod1,
                UnitPrice = 10,
                Quantity = 1
            );

            OrderItem oi2 = new OrderItem(
                Id = fflib_IDGenerator.generate(OrderItem.SObjectType),
                Product2Id = prod2.Id,
                Product2 = prod2,
                UnitPrice = 10,
                Quantity = 1
            );

            Order order = new Order();

            order = ((List<Order>) fflib_ApexMocksUtils.makeRelationship(
                List<Order>.class,
                new List<Order>{
                    order
                },
                OrderItem.OrderId,
                new List<List<OrderItem>>{
                    new List<OrderItem>{oi1, oi2}
                }
            ))[0];
        } catch (JSONException e) {
            Assert.fail('Should not throw a JSONException');
        }
    }
}