Not honoring @Nullable on null JSON fields

[luciofm]

Since https://github.com/rharter/auto-value-moshi/pull/76/files#diff-e17f3c4ec8512f83b9f9ec35523a3803L413 is not honoring @nullable properties when the actual JSON field is null, if the field is just missing, everything works fine.

Take this example:

@AutoValue
public abstract class WithNullableClass {
    public abstract String value();
    @Nullable public abstract NullableClass nullableClass();

    @AutoValue
    public static abstract class NullableClass {
        @Nullable public abstract String string();
    }
}

This will fail with {"value":"value","nullableClass":null} but not with {"value":"value"}

[ZacSweers]

Fixed in #95