title: Null value for Put Request in RestApi
body: <p>I'm coding Rest API with .NET Core. I am having a problem. I use Postman for test.
  For example; My variable: name, surname, age.</p>
  
  <pre><code>(Normally: ID:1, name: Amet, surname: Asar, age:30)
  </code></pre>
  
  <p>I'm using Put request.</p>
  
  <pre><code>{
    "ID": 1
    "name": "Ayhan",
    "surname":"Hara"
  }
  </code></pre>
  
  <p>But I don't send "age". So, it's saving null value for age.</p>
  
  <p>Like;</p>
  
  <pre><code>id=1, name=Ayhan, surname=Hara, age=null
  </code></pre>
  
  <p>How can check null variable? If the age value has not enter, don't change.</p>
  
  <pre><code>[HttpPut("{id}")]
  public ActionResult Update(int id, [FromBody]JsonPutDocument&lt;Customer_UserDTO&gt; customer_UserDTO)
  {
      if (!ModelState.IsValid)
          return BadRequest("İstenilen Datalar Düzgün Değil");
  
      var user = unitOfWork.customer_User.GetById(id);
      if (user == null)
          return NotFound("İlgili müşteri bulunamadı");
  
      var userToPut = mapper.Map&lt;Customer_UserDTO&gt;(user);
      customer_UserDTO.ApplyTo(userToPut, ModelState);
      mapper.Map(userToPut, user);
      unitOfWork.SaveChanges();
      return Ok(mapper.Map&lt;Customer_User&gt;(user));
  }
  </code></pre>