fix(change_detection): convert interpolated null values to empty strings

Fixes #3007

Closes #3271

modules/angular2/test/change_detection/change_detector_spec.ts

@@ -302,6 +302,15 @@ export function main() {
         expect(val.dispatcher.log).toEqual(['propName=BvalueA']);
       });
 
+      it('should output empty strings for null values in interpolation', () => {
+        var val = _createChangeDetector('interpolation', new TestData('value'));
+        val.changeDetector.hydrate(new TestData(null), null, null, null);
+
+        val.changeDetector.detectChanges();
+
+        expect(val.dispatcher.log).toEqual(['propName=BA']);
+      });
+
       it('should escape values in literals that indicate interpolation',
          () => { expect(_bindSimpleValue('"$"')).toEqual(['propName=$']); });
 

modules/angular2/test/core/compiler/integration_spec.ts

@@ -94,6 +94,19 @@ export function main() {
                });
          }));
 
+      it('should update text node with a blank string when interpolation evaluates to null',
+         inject([TestComponentBuilder, AsyncTestCompleter], (tcb: TestComponentBuilder, async) => {
+           tcb.overrideView(MyComp, new viewAnn.View({template: '<div>{{null}}{{ctxProp}}</div>'}))
+               .createAsync(MyComp)
+               .then((rootTC) => {
+                 rootTC.componentInstance.ctxProp = null;
+
+                 rootTC.detectChanges();
+                 expect(rootTC.nativeElement).toHaveText('');
+                 async.done();
+               });
+         }));
+
       it('should consume element binding changes',
          inject([TestComponentBuilder, AsyncTestCompleter], (tcb: TestComponentBuilder, async) => {
            tcb.overrideView(MyComp, new viewAnn.View({template: '<div [id]="ctxProp"></div>'}))