RIA/EF4 Entity property mapped to NOT NULL nvarchar - empty string

Background:

• Entity Framework 4
• Silverlight 4
• RIA services
• MSSQL Server 2008

I have an entity that has a String property named Description.

In database it maps to the NOT NULL NVARCHAR(200).

Problem:

When I try to insert a new row of that entity, this is what I do:

MyExampleEntity entity = new MyExampleEntity()
{
    Name = "example",
    Description = ""        // NOTE THIS LINE!
};

DatabaseContext db = new DatabaseContext();
db.MyExampleEntities.Add(entity);
db.SubmitChanges();

This, however, causes an exception saying "The Description field is required."

Question:

Should not the "empty string" be simply that - a string with zero characters?

I believe only Description = null should be treated as providing no value.

• Why is my string, which has a value (although its length is 0), considered to be as if I have omitted the value?
• On what level does this conversion happen? On RIA, on EF or in MSSQL?
• Is there a way to make a description have zero-length value when I set the Description to "" and cause an exception when Description = null (having no value)?