title: AutoMapper - set a destination property to null when source property is not available
body: <p>I have the following two objects -</p>
  
  <pre class="lang-cs prettyprint-override"><code>public class Customer
  {
      public Customer(string userName, string email)
      {
          this.UserName = userName;
          this.Email = email;
      }
  
      public string UserName { get; }
      public string Email { get; set; }
  }
  
  public class CustomerUpdate
  {
      public string Email { get; set; }
  }
  </code></pre>
  
  <p>I don't want to add a constructor in <code>Customer</code> to initialize <code>Email</code> only. Can I create a map from <code>CustomerUpdate</code> to <code>Customer</code> so that <code>UserName</code> is set to null?</p>
  
  <p>(I'm using AutoMapper 9.0.0)</p>