Newtonsoft.Json ignora vacío

class ContractResolver : CamelCasePropertyNamesContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var a = base.CreateProperty(member, memberSerialization);
        a.ShouldSerialize = p => filter(a.ValueProvider.GetValue(p));
        return a;
    }
    bool filter(object a)
    {
        if (a == null || "".Equals(a)) return false;
        var arr = a as ICollection;
        if (arr != null && arr.Count == 0) return false;
        return true;
    }
}

JsonConvert.SerializeObject (obj, Formatting.Indented, new JsonSerializerSettings ()
{     DateFormatString = "aaaa-MM-dd HH: mm: ss",     ContractResolver = new ContractResolver (), });