title: Automapper set destination property to null if source missed counterpart
body: <p>Please review example code. By some reason automapper set <code>y</code> value to null. In real world this happens when I map service dto to ef model.</p>
  <p>Below is complete example:
  output is:</p>
  <blockquote>
  <p>x = 1 y = System.Object</p>
  <p>x = 7 y =</p>
  </blockquote>
  <pre><code>class Program
  {
      static void Main(string[] args)
      {
          var a = new A();
          a.col.Add(new A1 { x = 7 });
         
          var b = new B();
          b.col.Add(new B1 { x = 1, y = new object() });
  
          Console.WriteLine($&quot;x = {b.col[0].x} y = {b.col[0].y}&quot;);
  
          a.Map(b);
  
          Console.WriteLine($&quot;x = {b.col[0].x} y = {b.col[0].y}&quot;);
      }
  }
  class A {
      public List&lt;A1&gt; col = new List&lt;A1&gt; { };
  }
  
  class A1
  {
      public int x;
  }
  
  
  class B
  {
      public List&lt;B1&gt; col = new List&lt;B1&gt; { };
  }
  
  class B1
  {
      public int x;
      public object y;
  }
  
  public static class Mapper
  {
      private static IMapper mapper;
  
      static Mapper()
      {
          var cfg = new MapperConfiguration(cfg =&gt;
          {
              cfg.CreateMap&lt;A, B&gt;();
              cfg.CreateMap&lt;A1, B1&gt;();
          });
  
          mapper = cfg.CreateMapper();
      }
      public static TDest Map&lt;TDest&gt;(this object source, TDest dest)
      {
          return mapper.Map(source, dest);
      }
  }
  </code></pre>