Fix empty string compilation

spec/regressions.js

@@ -115,5 +115,9 @@ describe('Regressions', function() {
     it("can pass through an already-compiled AST via compile/precompile", function() {
       equal(Handlebars.compile(new Handlebars.AST.ProgramNode([ new Handlebars.AST.ContentNode("Hello")]))(), 'Hello');
     });
+
+    it("can pass through an empty string", function() {
+      equal(Handlebars.compile('')(), '');
+    });
   }
 });

src/handlebars.yy

@@ -17,6 +17,7 @@ function stripFlags(open, close) {
 
 root
   : statements EOF { return new yy.ProgramNode($1); }
+  | EOF { return new yy.ProgramNode([]); }
   ;
 
 program