Generate more efficient code for empty literals

Original patch by Wolfgang Kluge:

  797173f676

src/emitter.js

@@ -710,6 +710,14 @@ PEG.compiler.emitter = function(ast) {
 
     literal: function(node, context) {
       var length = node.value.length;
+
+      if (length === 0) {
+        return formatCode(
+          "${resultVar} = '';",
+          { resultVar: resultVar(context.resultIndex) }
+        );
+      }
+
       var testCode = length === 1
         ? formatCode(
             "input.charCodeAt(pos) === ${valueCharCode}",

src/parser.js

@@ -280,15 +280,7 @@ PEG.parser = (function(){
         if (result0 !== null) {
           result1 = parse_literal();
           if (result1 === null) {
-            if (input.substr(pos, 0) === "") {
-              result1 = "";
-              pos += 0;
-            } else {
-              result1 = null;
-              if (reportFailures === 0) {
-                matchFailed("\"\"");
-              }
-            }
+            result1 = '';
             
           }
           if (result1 !== null) {
@@ -837,15 +829,7 @@ PEG.parser = (function(){
           pos3 = pos;
           result1 = parse_literal();
           if (result1 === null) {
-            if (input.substr(pos, 0) === "") {
-              result1 = "";
-              pos += 0;
-            } else {
-              result1 = null;
-              if (reportFailures === 0) {
-                matchFailed("\"\"");
-              }
-            }
+            result1 = '';
             
           }
           if (result1 !== null) {