Add overridable isFieldNull() method to JsonSerializer<T>

I've created a little 'sensitive data hiding' class that's basically a wrapper class with a custom` toString()` method.

To make this work with existing classes I registered a custom serialiser that unpacks the value and passes it on. This works great, and if the field is null, the `Include.NON_NULL` works great as well.

However, when the value inside the wrapper is null, it doesn't get picked up and it still serialises to `"fieldname":null`. To get around this I have to implement the `isEmpty()` method in the custom serialiser and use `Include.NON_EMPTY` on the class that uses the wrapper. This is not bad, but maybe it would be nice to either be able to specify when a field can be considered null with a `isFieldNull()` or something similar, so you can keep on using NON_NULL and not accidentally remove other fields for being an empty string for example.

I would understand if you'd rather keep it this way to not bloat the interface, but I thought I'd share the idea.