Ensure nil works as a variable name

lib/liquid/variable.rb

@@ -83,7 +83,6 @@ module Liquid
     end
 
     def render(context)
-      return ''.freeze if @name.nil?
       @filters.inject(context.evaluate(@name)) do |output, (filter_name, filter_args, filter_kwargs)|
         filter_args = evaluate_filter_expressions(context, filter_args, filter_kwargs)
         output = context.invoke(filter_name, output, *filter_args)

test/integration/tags/if_else_tag_test.rb

@@ -8,7 +8,11 @@ class IfElseTagTest < Minitest::Test
     assert_template_result('  this text should go into the output  ',
                            ' {% if true %} this text should go into the output {% endif %} ')
     assert_template_result('  you rock ?','{% if false %} you suck {% endif %} {% if true %} you rock {% endif %}?')
+  end
+
+  def test_literal_comparisons
     assert_template_result(' NO ','{% assign v = false %}{% if v %} YES {% else %} NO {% endif %}')
+    assert_template_result(' YES ','{% assign v = nil %}{% if v == nil %} YES {% else %} NO {% endif %}')
   end
 
   def test_if_else

test/integration/variable_test.rb

@@ -34,6 +34,10 @@ class VariableTest < Minitest::Test
     assert_equal 'false', Template.parse("{{ false }}").render!
   end
 
+  def test_nil_operations
+    assert_equal 'cat', Template.parse("{{ nil | append: 'cat' }}").render!
+  end
+
   def test_preset_assigns
     template = Template.parse(%|{{ test }}|)
     template.assigns['test'] = 'worked'