Best is not to concatenate the values before entering their values to a Session Storage, use a Keyed Array instead (key=>value). If the value is empty, the value for that key will also be empty. For more detail, see wCart Tutorial part 5. Add Session and Arrays to the Template. To set the values for the Keyed Array, see wCart Tutorial part 7. Apply Add to Cart Functionality
Rather than starting on your own, why not work through the whole tutorial, that way you will get an understanding of how easy it is without overthinking.