Avoid setting backend_name to empty string

When backend_name is empty string, then some code highlighters doesn't
work. Yeah, it's weird...

lib/asciidoctor/doctest/asciidoc_renderer.rb

@@ -1,4 +1,5 @@
 require 'active_support/core_ext/array/wrap'
+require 'active_support/core_ext/object/blank'
 require 'asciidoctor'
 require 'asciidoctor/converter/template'
 
@@ -12,7 +13,7 @@ class AsciidocRenderer
       attr_reader :backend_name, :converter, :template_dirs
 
       ##
-      # @param backend_name [#to_s] the name of the tested backend.
+      # @param backend_name [#to_s, nil] the name of the tested backend.
       #
       # @param converter [Class, Asciidoctor::Converter::Base, nil]
       #        the backend's converter class (or its instance). If not
@@ -33,10 +34,10 @@ class AsciidocRenderer
       # @raise [ArgumentError] if some path from the +template_dirs+ doesn't
       #        exist or is not a directory.
       #
-      def initialize(backend_name: '', converter: nil, template_dirs: [],
+      def initialize(backend_name: nil, converter: nil, template_dirs: [],
                      templates_fallback: false)
 
-        @backend_name = backend_name.freeze
+        @backend_name = backend_name.to_s.freeze.presence
         @converter = converter
         @converter ||= NoFallbackTemplateConverter unless template_dirs.empty? || templates_fallback
 

spec/asciidoc_renderer_spec.rb

@@ -11,12 +11,17 @@
 
     context 'with defaults' do
       subject { described_class.new }
-      it { is_expected.to have_attributes backend_name: '', converter: nil, template_dirs: nil }
+      it { is_expected.to have_attributes backend_name: nil, converter: nil, template_dirs: nil }
     end
 
     context 'with backend_name' do
       subject { described_class.new(backend_name: 'html5') }
       it { is_expected.to have_attributes backend_name: 'html5' }
+
+      context 'empty string' do
+        subject { described_class.new(backend_name: '') }
+        it { is_expected.to have_attributes backend_name: nil }
+      end
     end
 
     context 'with template_dirs' do