domains mishandle thrown nulls

This code throws user out of repl in 0.11.x:

```
bash$ node
> process.nextTick(function(){throw null})
undefined
> 
repl:1
(process.nextTick(function(){throw null})
                             ^
null
bash$
```

... as opposed to "throw 42" that doesn't.

Yeah, I know, throwing non-errors isn't nice, and shame on anybody who does that in real code.

*Note unrelated to this project: this pull request was created without any issues and/or previous discussions created anywhere. I don't expect to find anybody who gives a damn about that ([ref](https://github.com/nzakas/eslint/pull/470#issuecomment-31127265)).*