Check and insert 0 when a label is null

I have a formula like this
z = Convert.ToDouble(Label16.Text) + (((Convert.ToDouble(((x * Convert.ToDouble(Label16.Text)) + (x * Convert.ToDouble(Label22.Text)) + (x * Convert.ToDouble(Label23.Text)) + (x * Convert.ToDouble(Label24.Text)) / gvObjective.Rows.Count))) * 0.2)) / 100;
now i have to use 0 when there is nothing in the label how can i do it??
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