`GrantedAuthority.getAuthority()` should be marked `@Nullable`

[rwinch]

The javadoc of GrantedAuthority.getAuthority() states that the return value an be null, but it is not marked as @Nullable. We should update it to be marked as @Nullable and update the build to account for this.

[therepanic]

Hi, @rwinch. Can I work on this?

[rwinch]

Thank you for volunteering @therepanic! The issue is yours