How to count the NULL or balnk line of column

Question

Hi!

I write these code to filter some data. Then on the 11th column there the value "1" to make counter and show it on this value" lblAusenteTotal.Text".
What I need to learn?
How can I make it to counter blank or NULL line of the 11th column.

int sum = 0;

            for (int i = 0; i < dvgDatas.Rows.Count; ++i)

            {

                sum += Convert.ToInt32(dvgDatas.Rows[i].Cells[11].Value);

                lblResult.Text = " " + sum.ToString();