largest-series-product: Add cases where all spans contain a 0
largest-series-product: Add cases where all spans contain a 0

In these cases, the result should be 0 as well. This guards against
solutions that assume the minimum is 1.

exercism/xcsharp,exercism/xcsharp,ErikSchierboom/xcsharp,GKotfis/csharp,robkeim/xcsharp,GKotfis/csharp,robkeim/xcsharp,ErikSchierboom/xcsharp