int property: Default value becomes 0 in text box although I set default values to null

Description

Steps to reproduce

1. Set a default value to null in page/block model.

   [Display(GroupName = SystemTabNames.Content)]

   [Range(0,10)]

   public virtual int? NetworkCatalogDepth { get; set; }

   public override void SetDefaultValues(ContentType contentType)

   {

   base.SetDefaultValues(contentType);

   NetworkCatalogDepth = null;

   }

2. Create a new block with this property from a Content Area.

Expected:
The text box is empty, like when I create a new block from the Assets pane.

Actual:
The default value in the text box becomes 0 for an int property.