Support null for Map<string, obj>

[katatunix]

I'm trying to serialize/deserialize a value of type Map<string, obj>, using the allowUntyped config already.

Everything is fine except when some obj is null. For example, deserializing { "hello": null } will throw System.NullReferenceException; the same for serializing Map [ ("hello", null) ].

Could you please support this particular case?