WebAPI: Null values not suppressed by Default XML Serializer

I tried using advice from other articles here on StackOverflow regarding this issue, but no dice. I am unable to suppress my null properties from Xml Serialization.

Here's my fairly simple class. As you can see I am trying to use declarative attributes, as well as the ShouldSerialize{variable} technique but neither of these is effective in suppressing these from the Xml.

public class Practice
{
#pragma warning disable 0649
    public int id;
    public string name;
    public string sharedId;
    public string sharedIdType;
    
    [XmlElement(IsNullable = false)]
    public List<Doctor> doctors;

    [XmlElement(IsNullable = false)]
    public List<Employee> employees;

    public bool ShouldSerializedoctors()
    {
        return !(doctors == null);
    }

    public bool ShouldSerializeemployees()
    {
        return !(employees == null);
    }
#pragma warning restore 0649
}

Here is a snippet from the the resulting Xml. As you can see, doctors was serialized into the Xml even though it is null. This is a WebAPI project running on Framework 4.5.2. Any idea what I'm doing wrong?

<ArrayOfPractice xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/rater8.Common.Models">
   <Practice>
       <doctors i:nil="true"/>
       <employees>
           <Employee>
              <id>49</id>

BTW - I was able to suppress the null values from Json serialization using:

config.Formatters.JsonFormatter.SerializerSettings =
    new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

Here is some additional information:

By adding the following statement into WebApiCofig.Register I now see the suppression of the nil elements.

config.Formatters.XmlFormatter.UseXmlSerializer = true;

Here is the full code for this method:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services

        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );

        config.Formatters.JsonFormatter.SerializerSettings =
             new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

        config.Formatters.XmlFormatter.UseXmlSerializer = true;

        GlobalConfiguration.Configuration.Formatters.JsonFormatter.MediaTypeMappings.Add
            (new QueryStringMapping("format", "json", "application/json"));
        GlobalConfiguration.Configuration.Formatters.XmlFormatter.MediaTypeMappings.Add
            (new QueryStringMapping("format", "xml", "application/xml"));
    }
}

So this solves the original problem description but this raises some additional questions.

1. What is the default WebAPI Xml Serializer, other than XmlSerializer?

2. I note that the structure of the Xml emitted by XmlSerializer is different (aside from the omission of the nil elements) than that which is emitted by the default WebAPI serializer. Is this going to cause problems on the attempt to deserialize the Xml to its class representation at the receiving end?