title: JSON.NET NullValueHandling in JsonPropertyAttribute not working as expected
body: <p>I have a class like so:</p>
  
  <pre><code>[JsonObject]
  public class Condition
  {
      [JsonProperty(PropertyName = "_id")]
      public string Id { get; set; }
  
      [JsonProperty(PropertyName = "expressions", NullValueHandling = NullValueHandling.Ignore)]
      public IEnumerable&lt;Expression&gt; Expressions { get; set; }
  
      [JsonProperty(PropertyName = "logical_operation")]
      [JsonConverter(typeof(StringEnumConverter))]
      public LogicOp? LogicalOperation { get; set; }
  
      [JsonProperty(PropertyName = "_type")]
      [JsonConverter(typeof(AssessmentExpressionTypeConverter))]
      public ExpressionType Type { get; set; }
  }
  </code></pre>
  
  <p>However, when the <code>Expressions</code> property is null, and I serialize the object like so:</p>
  
  <pre><code> var serialized = JsonConvert.SerializeObject(condition, Formatting.Indented);
  </code></pre>
  
  <p>... the text of the Json string has the line:</p>
  
  <pre><code>"expressions": null
  </code></pre>
  
  <p>My understanding is that this should not happen.  What am I doing wrong?</p>