Write empty Nullable types as null

[mrow84]

Apologies if I've misunderstood something, but it seems like Nullable values don't get written as JSON null if they are empty. Sorry for not setting up a pull request, but I think adding something like this should fix it:

function _print{T}(io::IO, state::State, n::Nullable{T})
    if isnull(n)
        Base.print(io, "null")
    else
       _print(io, state, get(n))
    end
end

[TotalVerb]

This should hopefully be easy to add, without adding more _print overloads, once #151 is finished.