Update license conversion

[Korusuke]

Return unknown if no license found and test for the same

Fixes #14

[reneeotten]

@Korusuke this seems to work as intended, but the return-statement becomes a bit hard to decipher... not sure if that's a real issue but readability does count to a certain point. @Steap what do you think?

[Steap]

It is not super elegant, but having an "if/else" may not be that much better. I think it is down to personal preference here.

[reneeotten]

okay, let's merge this then