Comment 3 for bug 1549393

The goal of using B::ISBN was to make sure we pass actual ISBNs. I'd tend to toss this at B&T ask they skip emtpy <Key>s (because not every bug is ours to fix) but I suspect they may be restricted by their choice of tools.

I think it would be reasonable to use grep in front of the maps on lines 185-187 to remove empty strings, which would solve the empty-key problem, in the least. But I don't think we should stop validating ISBNs generally.