title: class-transformer Exclude undefined properties
body: <p>Exclude undefined or null properties from the class. this is actual nature but I need a decorator who can ignore this</p>
  
  <pre><code>import {Expose, plainToClass} from "class-transformer";
  
  class User {
      @Expose() id: number;
      @Expose() firstName: string;
      @Expose() lastName: string;
  }
  
  const fromPlainUser = {
    unkownProp: 'hello there',
    firstName: 'Umed',
    lastName: 'Khudoiberdiev',
  }
  
  console.log(plainToClass(User, fromPlainUser, { excludeExtraneousValues: true }))
  
  // User {
  //   id: undefined,
  //   firstName: 'Umed',
  //   lastName: 'Khudoiberdiev'
  // }
  </code></pre>