largest-series-product: Test all combinations of corner cases
largest-series-product: Test all combinations of corner cases

The original tests had tests for "" and 0.
This is when both inputs are the boundary condition.
Now we should also test when only one input is at the boundary.
So we should test a non-empty string with 0, and "" with a nonzero span.

These are consistent with the values defined in
https://github.com/exercism/x-common/blob/master/largest-series-product.json

exercism/xperl5,exercism/xperl5,exercism/xperl5