Removes code duplication (code climate)

No gain but it's "better": https://codeclimate.com/github/Mogztter/asciidoctor/compare/code-climate
```ruby
spec.delete_if { |_, v| v.nil_or_empty? }
```

`nil_or_empty?` is not working because this method is undefined on some types. For example: `undefined method 'nil_or_empty?' for :strong:Symbol (NoMethodError)`

I think it's safe to use `nil?`