Access original cell value from custom tag

[gknyihar]

I want to create a new custom tag that replaces all null values with #N/A. I can replace cell values from the tag but cannot distinguish empty strings from null because it outputs the same value to the cell. So, I need to inspect the original values accessible from the ProcessingContext Value property, but I cannot map the cell to the proper value.

public class NullableTag : OptionTag
{
    public override void Execute(ProcessingContext context)
    {
        var dataSource = context.Value as IDataSource;
        var range = context.Range;

        range.Cells().ToList().ForEach(cell =>
        {
            var row = range.FindRow(r => r.Contains(cell));
            var values = dataSource.GetValue(row);

            var value = ... // get value from values by cell address

            if (value == null)
            {
                cell.FormulaA1 = "=NA()";
            }

        });
    }

}

Could you point me in the right direction?