Removes code duplication (code climate)

No gain but it's "better": https://codeclimate.com/github/Mogztter/asciidoctor/compare/code-climate
IMHO it’d be better to add `nil_or_empty?` alias to the Symbol class. I’m wondering why it’s not there already.

To [asciidoctor/core_ext/object/nil_or_empty](https://github.com/asciidoctor/asciidoctor/blob/v1.5.1/lib/asciidoctor/core_ext/object/nil_or_empty.rb):
```ruby
class Symbol
  alias :nil_or_empty? :empty? unless respond_to? :nil_or_empty?
end
```