null Abstract @:to

The following error was reported by an user https://github.com/HaxeFoundation/neko/issues/54
While investigating it, I found that what happens is that when doing:

```
var o : Null = null;
trace("Result = "+o);
```

Translate to:

```
trace("Result = "+Std.string(o.toFloat());
```

And the toFloat() performs a `null + 0.` addition which triggers an exception.

I could easily fix it by patching UInt, but I wonder if we should not find another way. For instance for all nullable abstracts, a toString() would check first for (if( o == null ) "null" else o.toString()).

What do you think @Simn ?