Don't error out on empty Sass function arglists.

This also means we've got to add a Nil type for Sass.

lib/sass/constant.rb

@@ -207,7 +207,10 @@ module Sass
       #++
       def operationalize(value, constants)
         value = [value] unless value.is_a?(Array)
-        if value.length == 1
+        case value.length
+        when 0
+          Sass::Constant::Nil.new
+        when 1
           value = value[0]
           if value.is_a? Array
             operationalize(value, constants)
@@ -216,7 +219,7 @@ module Sass
           else
             Literal.parse(value)
           end
-        elsif value.length == 2
+        when 2
           if value[0] == :neg
             Operation.new(Sass::Constant::Number.new('0'), operationalize(value[1], constants), :minus)
           elsif value[0] == :const
@@ -224,7 +227,7 @@ module Sass
           else
             raise SyntaxError.new("Constant arithmetic error")
           end
-        elsif value.length == 3
+        when 3
           Operation.new(operationalize(value[0], constants), operationalize(value[2], constants), value[1])
         else
           if SECOND_ORDER.include?(value[1]) && FIRST_ORDER.include?(value[3])

lib/sass/constant/literal.rb

@@ -4,6 +4,7 @@ module Sass::Constant; class Literal; end; end; # :nodoc:
 require 'sass/constant/string'
 require 'sass/constant/number'
 require 'sass/constant/color'
+require 'sass/constant/nil'
 
 class Sass::Constant::Literal # :nodoc:
   # The regular expression matching numbers.

lib/sass/constant/nil.rb

@@ -0,0 +1,9 @@
+require 'sass/constant/literal'
+
+module Sass::Constant # :nodoc:
+  class Nil < Literal # :nodoc:
+    def to_s
+      ''
+    end
+  end
+end

test/sass/engine_test.rb

@@ -132,8 +132,8 @@ END
   end
 
   def test_default_function
-    assert_equal("foo {\n  bar: url(foo.png); }\n",
-                 render("foo\n  bar = url(foo.png)\n"));
+    assert_equal("foo {\n  bar: url(foo.png); }\n", render("foo\n  bar = url(foo.png)\n"));
+    assert_equal("foo {\n  bar: url(); }\n", render("foo\n  bar = url()\n"));
   end
 
   def test_basic_multiline_selector