Display Null if datagridview Column is empty

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C#

DataGridView

I have a DataGridView with few column cells. all i want is to check an empty cells and write null by default. if some of the columns are not filled., i want a default null text to the specific cells that are committed

I tried to create something like this but it is not working as i don't know how i can take it further

if (Gridview_Output.Rows.Count > 0)
            {

                foreach (DataRow dr in oDataSet.Tables[0].Rows)
                {

                    string _Car = dr[1].ToString();
                    string _Dealer = dr[2].ToString();
                    string _Model = dr[4].ToString();

                    if (_Car.Length == 0)
                    {
                        _Car = "NULL";
                    }

                    if (_Dealer.Length == 0)
                    {
                        _Dealer = "NULL";
                    }

                    if (_Model.Length == 0)
                    {
                        _Model = "NULL";
                    }
                }

            }