Null String in Database returns String.Empty instead of Null when mapping into anonymous class

[Atri15]

The issue is similar as #321

I've set the config value to null
BLToolkit.Common.Configuration.NullableValues.String = null;
but it does not work for select to the anonymous class.

public partial class Table1{
...
[MapField("string_value"), Nullable]
public string    StringValue        { get; set; } // nvarchar(-1)
...
}

var query =
        d.GetTable<Table1>()
               .Where(x => x.StringValue == null)
               .Select(x => new { x, x.StringValue });

var queryData = query.First();

And result in json is:

queryData = 
{
  "x": {
 ...
    "StringValue": null,
...
  },
  "StringValue": ""
}

But expected:

queryData = 
{
  "x": {
 ...
    "StringValue": null,
...
  },
  "StringValue": null
}