Cell has
FormattedValue property which gets formatted representation of object contained as cell's value, i.e. if cell.Value is null you'll see empty string and so on.
foreach(DataGridViewRow row in yourDatagridView.Rows)
{
if(string.IsNullOrWhiteSpace(row.Cells["many"].FormattedValue))
{
MessageBox("BOOM! Many is empty",":(");
return;
}
}