const string MyGuid = nameof(MyGuid);
var dt = new DataTable();
dt.Columns.Add(new DataColumn(MyGuid, typeof(Guid)));

var row1 = dt.NewRow();
row1[MyGuid] = DBNull.Value;

dt.Rows.Add(row1);

var row2 = dt.NewRow();
row2[MyGuid] = Guid.NewGuid();

dt.Rows.Add(row2);

foreach(DataRow row in dt.Rows) {
    var obj = row[MyGuid];
    var res = Convert.IsDBNull(obj) ? null : (Guid?)obj;
    Console.WriteLine(res);
}