fix(datepicker): consider empty string inputs as valid

Fixes #1588

Closes #1637

src/datepicker/datepicker-input.spec.ts

@@ -309,6 +309,22 @@ describe('NgbInputDatepicker', () => {
            tick();
            expect(form.control.invalid).toBeTruthy();
          }));
+
+      it('should consider empty strings as valid', fakeAsync(() => {
+           const fixture = createTestCmpt(`<form><input ngbDatepicker [(ngModel)]="date" name="dp"></form>`);
+           const inputDebugEl = fixture.debugElement.query(By.css('input'));
+           const form = fixture.debugElement.query(By.directive(NgForm)).injector.get(NgForm);
+
+           inputDebugEl.triggerEventHandler('change', {target: {value: '2016-09-10'}});
+           fixture.detectChanges();
+           tick();
+           expect(form.control.valid).toBeTruthy();
+
+           inputDebugEl.triggerEventHandler('change', {target: {value: ''}});
+           fixture.detectChanges();
+           tick();
+           expect(form.control.valid).toBeTruthy();
+         }));
     });
 
   });

src/datepicker/datepicker-input.ts

@@ -186,7 +186,7 @@ export class NgbInputDatepicker implements OnChanges,
 
   manualDateChange(value: string) {
     this._model = this._service.toValidDate(this._parserFormatter.parse(value), null);
-    this._onChange(this._model ? {year: this._model.year, month: this._model.month, day: this._model.day} : value);
+    this._onChange(this._model ? this._model.toStruct() : (value === '' ? null : value));
     this._writeModelValue(this._model);
   }
 

src/datepicker/ngb-date.ts

@@ -40,5 +40,7 @@ export class NgbDate {
     }
   }
 
+  toStruct() { return {year: this.year, month: this.month, day: this.day}; }
+
   toString() { return `${this.year}-${this.month}-${this.day}`; }
 }