Simplified printing of Nullable objects

Before:

```julia
julia> Nullable{Int}()
Nullable{Int}()

julia> Nullable{Int}(1)
Nullable(1)
```

After:

```julia
julia> Nullable{Int}()
NULL

julia> Nullable{Int}(1)
1
```

I'm not sure how we should Nullables, but this simplified printing makes working with Nullables much more enjoyable (when you already know that you're dealing with a potential source of Nullables).