FallbackOnNull should support Strings

[josh-burton]

I realise that FallbackOnNull seems to be geared towards primitives, but supporting any type would be great.

Maybe a separate adapter (DefaultOnNull) could work, where a default value for any type when it is null.

[serj-lotutovici]

I thought that moshi already had this in their adapters artifact, but no.
This should be a good addition, altho I don't think that adding an annotation would be a good approach. Maybe smth like DefaultOnNullJsonAdapterFactory.create(Type type, T fallback).

Wanna submit a PR?

[josh-burton]

So would that be a global default for a type? That would be handy but I think I would also want to be able to specify that behaviour for a single field rather than every field as well.

[serj-lotutovici]

The annotation api is pretty limited in java. A fallback for strings is possible, and for that, we can just extend the existing FallbackOnNull, but for objects, it's trickier. There's no way, that I know of, that would allow settings an object value as an annotation param.