`Void` type annotations without specifying `null` serialized value

[rayningw]

Currently, annotations of the Void type needs to specify null as the serialized value, e.g.

newtype Localizable = Void;

struct MyStruct {
    @Localizable null
    String name;
}

Since there can only be one value for a Void type, it would be good to make this implicit and allow annotations like so:

@Localizable
String name;

[timbod7]

Fixed with 48895ad