[REG2.063] Issue 10268 - std.typecons.Nullable!JSONValue - error instantiating

http://d.puremagic.com/issues/show_bug.cgi?id=10268

I found that the released qualified constructor feature in 2.063 + inout doesn't work ([bug10313](http://d.puremagic.com/issues/show_bug.cgi?id=10313)), so use a workaround for the bug.