title: Need to change default date when not selecting a date in matdatepicker
body: <p>When I am not selecting a date in matdatepicker (angular2), it's taking the default value as "01/01/1970", whereas I need to pass 'null' to the method. Can I change the default value somehow?</p>
  
  <p>Code I am using is as below:</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;div class="div1"&gt;
     &lt;form (ngSubmit)="getEventsByDateRange(f)" #f="ngForm" &gt;
       &lt;label style="float:left;padding:7px;"&gt; From : &lt;/label&gt;
        &lt;mat-form-field style="float:left;padding:7px;"&gt;
          &lt;input type="date" matInput [matDatepicker]="picker1" placeholder="Choose a date" name="fromDate" 
          [(ngModel)]="eventobj.fromDate" #eventDate="ngModel"  required&gt;
          &lt;mat-datepicker-toggle matSuffix [for]="picker1"&gt;&lt;/mat-datepicker-toggle&gt;
          &lt;mat-datepicker #picker1&gt;&lt;/mat-datepicker&gt;
        &lt;/mat-form-field&gt;
        &lt;label style="float:left;padding:7px;"&gt; To : &lt;/label&gt;
        &lt;mat-form-field style="float:left;padding:7px;"&gt;
          &lt;input type="date" matInput [matDatepicker]="picker2" placeholder="Choose a date" name="toDate" 
          [(ngModel)]="eventobj.toDate" #eventDate="ngModel"  required&gt;
          &lt;mat-datepicker-toggle matSuffix [for]="picker2"&gt;&lt;/mat-datepicker-toggle&gt;
          &lt;mat-datepicker #picker2&gt;&lt;/mat-datepicker&gt;
        &lt;/mat-form-field&gt;
        &lt;button mat-raised-button class="btn btn-info btn-sm"&gt; Submit &lt;/button&gt;
     &lt;/form&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>TS:</p>
  
  <pre><code>eventobj = {
  fromDate: "Date",
  toDate: "Date",
  }
  
   getJobsByDateRange() {
      let fromDate1=new Date(this.eventobj.fromDate)
      let toDate1=new Date(this.eventobj.toDate)
  
      alert("from :: "+fromDate1+" to :: "+toDate1)
   }
  </code></pre>
  
  <p>If I am clicking on submit button directly then the alert message shows :</p>
  
  <blockquote>
    <p>from :: Thu Jan 01 1970 05:30:00 GMT+0530 (India Standard Time) to :: Thu Jan 01 1970 05:30:00 GMT+0530 (India Standard Time)</p>
  </blockquote>