Use @NonNullApi for null-safety

[pkubowicz]

Summary

Use @NonNullApi annotation as Spring Core did.

Actual Behavior

Many parts of the API like ClaimAccessor return null, so it's easy to cause NPE if you don't read JavaDoc carefully.

Expected Behavior

Use @NonNullApi and @Nullable at least for part of the API.

Configuration

Version

All versions up to today's master.