null reference exception when options class property has not setter.

For Excample:

```CSharp
  class Program
  {

    private class Options
    {
      [Value( 0, MetaName = "Test", Default = 0 )]
      public int TestValue{ get; }
    }

    static void Main( string[] args )
    {
      Parser.Default.ParseArguments<Options>( args );//throw NullReferenceException.
    }
  }
```