Add undefined and null literal support

This adds the UndefinedLiteral and NullLiteral to AST.

Fixes #990

lib/handlebars/compiler/ast.js

@@ -101,6 +101,18 @@ var AST = {
       this.value = bool === 'true';
   },
 
+  UndefinedLiteral: function(locInfo) {
+    this.loc = locInfo;
+    this.type = 'UndefinedLiteral';
+    this.original = this.value = undefined;
+  },
+
+  NullLiteral: function(locInfo) {
+    this.loc = locInfo;
+    this.type = 'NullLiteral';
+    this.original = this.value = null;
+  },
+
   Hash: function(pairs, locInfo) {
     this.loc = locInfo;
     this.type = 'Hash';

lib/handlebars/compiler/compiler.js

@@ -273,6 +273,14 @@ Compiler.prototype = {
     this.opcode('pushLiteral', bool.value);
   },
 
+  UndefinedLiteral: function() {
+    this.opcode('pushLiteral', 'undefined');
+  },
+
+  NullLiteral: function() {
+    this.opcode('pushLiteral', 'null');
+  },
+
   Hash: function(hash) {
     var pairs = hash.pairs, i, l;
 

lib/handlebars/compiler/printer.js

@@ -124,6 +124,14 @@ PrintVisitor.prototype.BooleanLiteral = function(bool) {
   return "BOOLEAN{" + bool.value + "}";
 };
 
+PrintVisitor.prototype.UndefinedLiteral = function() {
+  return 'UNDEFINED';
+};
+
+PrintVisitor.prototype.NullLiteral = function() {
+  return 'NULL';
+};
+
 PrintVisitor.prototype.Hash = function(hash) {
   var pairs = hash.pairs;
   var joinedPairs = [];

lib/handlebars/compiler/visitor.js

@@ -110,6 +110,8 @@ Visitor.prototype = {
   StringLiteral: function(/* string */) {},
   NumberLiteral: function(/* number */) {},
   BooleanLiteral: function(/* bool */) {},
+  UndefinedLiteral: function(/* literal */) {},
+  NullLiteral: function(/* literal */) {},
 
   Hash: function(hash) {
     this.acceptArray(hash.pairs);