Support nil object reference

lib/hamlit/object_ref.rb

@@ -3,6 +3,8 @@ module Hamlit
     class << self
       def parse(args)
         object, prefix = args
+        return {} unless object
+
         suffix = underscore(object.class)
         {
           'class' => [prefix, suffix].compact.join('_'),

test/haml/template_test.rb

@@ -48,7 +48,7 @@ class TemplateTest < Haml::TestCase
     #'whitespace_handling',
     'original_engine',
     'list',
-    #'helpful',
+    'helpful',
     'silent_script',
     'tag_parsing',
     #'just_stuff',

test/hamlit/engine/attributes_test.rb

@@ -193,6 +193,7 @@ describe Hamlit::Engine do
       - dynamic = 'dynamic'
       %a.static#static[foo, 'pre']{ id: dynamic, class: dynamic }
     HAML
+    assert_render(%q|.static#static[nil]|, %Q|<div class='static' id='static'></div>\n|)
   end
 
   describe 'engine options' do