I can use the ExcludeDefaultValues to exclude json serializing of null values in strings. But I also want "" to be excluded. I do not have access to the model to set custom attributes. Is there a way to create a kind of override in the string serializer?
In Newtonsoft I can do:

			protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
			{
				var property = base.CreateProperty(member, memberSerialization);
				if (property.PropertyType == typeof(string))
				{
					property.DefaultValue = "";
				}

				return property;
			}

but I want to use SS if possible.
Thanks.

Could you use a custom serialization function for the model in question?

JsConfig<Model>.OnSerializingFn = obj =>
{
    if (obj.ModelProperty?.Length == 0)
    {
        obj.ModelProperty = null;
    }

// Repeat for other properties
}

I believe that would stack with ExcludeDefaultValues but I'm sure @mythz will correct me with a better answer :).

No actually. It is not for 1 specific model, but for every object that I want to serialize. So I need a more generic approach that works for every object given where a string is used (hierarchical)

Out of pure coincidence, I was just looking for something to do this for me on only my inbound JSON, but I believe mythz solution here might be what you need? Or a starter, at least?

There's no JS configuration that lets you configure this behavior, the only 2 generic solutions I can think of is similar to @daleholborow link of registering a Response Filter that uses reflection to generically go through each property and replace each empty string with null.

The other solution is to add a ShouldSerialize() API on POCOs you want this behavior on to ignore any string properties with null or empty values, e.g:

public class Poco {
  public bool? ShouldSerialize(string name) => this.IgnoreEmptyStrings(name);
}

public static class MyExt
{
    public static bool? IgnoreEmptyStrings<T>(this T obj, string name)
    {
        var accessor = TypeProperties.Get(typeof(T)).GetAccessor(name);
        return accessor?.PropertyInfo.PropertyType == typeof(string)
            ? !string.IsNullOrEmpty((string)accessor.PublicGetter(this))
            : (bool?)null;
    }
}