title: FormGroup thinks it's required when no validators are specified
body: <p>I have a form with a nested FormGroup:</p>
  
  <pre><code>this.form = this.formBuilder.group({
    kitType: [
      '', [
        Validators.required,
      ]
    ],
    diagnosisCode: [
      '', [
        Validators.required,
      ]
    ],
    delayKitShipmentDate: new FormGroup({
      year: new FormControl(''),
      month: new FormControl({ value: '', disabled: true }),
      day: new FormControl({ value: '', disabled: true }),
    })
  });
  </code></pre>
  
  <p>The <code>delayKitShipmentDate</code> is optional, and does not have any validators specified on the controls, yet my form is reporting <code>invalid</code> unless I specify values for the year, month and day controls.</p>
  
  <p>What am I doing wrong?</p>