Document validation for nil value

Add a test to document ConfigValidator's behaviour when passed a nil
value against a schema expecting a string when `allowed_empty` is set to
`true`.

This is useful to understand due to the way that Ruby's YAML library
parser returns `nil` if nothing is specified after the member's key
name:

```ruby
> YAML::load('base:')
=> {"base"=>nil}

> YAML::load('base: ')
=> {"base"=>nil}

> YAML::load('base: ""')
=> {"base"=>""}
```