How to display text in DataGridViewComboBoxColumn when the databound value is null
//attach in code or via designer:
dataGridView1.CellFormatting += new DataGridViewCellFormattingEventHandler(dataGridView1_CellFormatting);


    //example implementation:
    void dataGridView1_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
    {
        if (e.ColumnIndex == Column1.Index && e.Value==null)//where Column1 is your combobox column
        {
            e.Value = "Empty";
            e.FormattingApplied = true;
        }
    }