title: Prevent angular form from submitting if input is blank
body: <p>I have an Angular form inside a ng2 popup:</p>
  
  <pre><code>&lt;popup&gt;
    Sign up for our Newsletter!     &lt;form #f="ngForm" (ngSubmit)="onSubmit()" novalidate&gt;
  &lt;/button&gt;         &lt;input type="email"/&gt;
        &lt;button&gt;Submit&lt;/button&gt;
      &lt;/form&gt;
  &lt;/popup&gt;
  &lt;button class="submit" (click)="ClickButton()"&gt;Sign up for our Newsletter &lt;/button&gt;
  </code></pre>
  
  <p>here is the onClick event function:</p>
  
  <pre><code>constructor(private popup:Popup) { }
    testAlert() {   ClickButton(){
      alert("Newsletter event works");    this.popup.options = {
    widthProsentage: 15,
    showButtons: false,
    header: "Sign up for our Newsletter!",
  }
  this.popup.show(this.popup.options);
    }   
  </code></pre>
  
  <p>It works fine but I am able to submit her even if the input is blank, how can I make so that it does not submit if it is clicked empty</p>
  
  <p>I tried using RegEx but it did not work</p>