set validation: validate as fine if we have null instead of iterable

Rationale: now I can skip/abort the validation conditionally such as with the
following:

```
  in: ->
    if @get('country') == 'CA'
      ['ON', 'QC', 'NS', 'NB', 'MB', 'BC', 'PE', 'SK', 'AB', 'NL', 'NT', 'YT', 'NU']
    else
      null
```

What do you think of this? This functionality would definitely be nice but maybe you have a different idea of how it should be implemented.