Epic productSlug may be null

[moonstar-x]

🐛 Describe the Bug

This happened today for a Paladins dlc offer, the productSlug was null. Apparently it is the urlSlug that should be used instead.

For now it's not that big of a deal because it happens with only dlcs (I believe) and in most cases the productSlug does come with the actual slug expected.

[moonstar-x]

Update:

Some of these games also return a null productSlug:

[moonstar-x]

This should be fixed in #26. Further testing might be necessary.