title: How to post NULL value from FormGroup
body: <p>I've got the following model (simplified):</p>
  <pre><code>export interface IClient {
   address?: IClientAddress;
  }   
  
  export interface IClientAddress {
   streetAddress?: string;
   coordinates?: string;
  }
  </code></pre>
  <p>And the following FormGroup:</p>
  <pre><code>this.clientForm = this.fb.group({
    address: this.fb.group({
      streetAddress: [{ value: '' }],
      coordinates: [{ value: '' }]
    })
  });
  </code></pre>
  <p>Now when null 'address' field is received from backend, this is being sent back:</p>
  <pre><code>{ address: {streetAddress: null, coordinates: null} }
  </code></pre>
  <p>How can I make FormControl to send this?</p>
  <pre><code>{ address: null }
  </code></pre>