Return empty collection instead of null

[Tomas Laubr]

It's a good practice to return empty collection instead of null in methods like JWTClaimsSet.getAudience(). Otherwise it's harder to work with the library.

[Vladimir Dzhuvinov]

Yes, that's good idea to prevent NPEs and other hassle :)

[Vladimir Dzhuvinov]

Changed in commit e058460, soon on Maven Central as v 4.15