fix(forms): default the initial value of Control to null

modules/angular2/src/forms/directives/ng_form.ts

@@ -74,7 +74,7 @@ export class NgForm extends ControlContainer implements Form {
   addControl(dir: NgControl): void {
     this._later(_ => {
       var container = this._findContainer(dir.path);
-      var c = new Control("");
+      var c = new Control();
       setUpControl(c, dir);
       container.addControl(dir.name, c);
       c.updateValidity();

modules/angular2/src/forms/directives/ng_model.ts

@@ -36,7 +36,7 @@ const formControlBinding = CONST_EXPR(new Binding(NgControl, {toAlias: forwardRe
   exportAs: 'form'
 })
 export class NgModel extends NgControl {
-  _control = new Control("");
+  _control = new Control();
   _added = false;
   update = new EventEmitter();
   model: any;

modules/angular2/src/forms/model.ts

@@ -144,7 +144,7 @@ export class AbstractControl {
 export class Control extends AbstractControl {
   _onChange: Function;
 
-  constructor(value: any, validator: Function = Validators.nullValidator) {
+  constructor(value: any = null, validator: Function = Validators.nullValidator) {
     super(validator);
     this._value = value;
     this.updateValidity({onlySelf: true});

modules/angular2/test/forms/model_spec.ts

@@ -19,6 +19,12 @@ import {ObservableWrapper} from 'angular2/src/facade/async';
 export function main() {
   describe("Form Model", () => {
     describe("Control", () => {
+      it("should default the value to null", () => {
+        var c = new Control();
+        expect(c.value).toBe(null);
+        expect(c.validator).toBe(Validators.nullValidator);
+      });
+
       describe("validator", () => {
         it("should run validator with the initial value", () => {
           var c = new Control("value", Validators.required);