You can create your own custom model metadata provider like this:
public class CustomModelMetadataProvider : DataAnnotationsModelMetadataProvider
{
protected override ModelMetadata CreateMetadata(IEnumerable<System.Attribute> attributes, System.Type containerType, System.Func<object> modelAccessor, System.Type modelType, string propertyName)
{
var modelMetadata = base.CreateMetadata(attributes, containerType, modelAccessor, modelType, propertyName);
if (string.IsNullOrEmpty(propertyName)) return modelMetadata;
if (modelType == typeof(String))
modelMetadata.ConvertEmptyStringToNull = false;
return modelMetadata;
}
}
Then register it in your app_start:
ModelMetadataProviders.Current = new CustomModelMetadataProvider();