Make possible users easily to change the behavior of replacing the null value with empty string.

Workaround: 
DataTable table = new DataTable("table");
table.Columns.Add("ID", typeof(int));
DataColumn col = table.Columns.Add("Name", typeof(string));
col.AllowDBNull = false;
col.DefaultValue = string.Empty;
table.ColumnChanging += new DataColumnChangeEventHandler(table_ColumnChanging);
this.radGridView1.DataSource = table;

private void table_ColumnChanging(object sender, DataColumnChangeEventArgs e)
{
    if (!e.Column.AllowDBNull && (e.ProposedValue == DBNull.Value || e.ProposedValue == null) && e.Column.DataType == typeof(string))
    {
        e.ProposedValue = string.Empty;
    }
}