Update reactive auditing documentation considering `Authentication` can be `null`.

Closes #3413

src/main/antora/modules/ROOT/pages/auditing.adoc

@@ -104,6 +104,7 @@ class SpringSecurityAuditorAware implements ReactiveAuditorAware<User> {
   public Mono<User> getCurrentAuditor() {
 
     return ReactiveSecurityContextHolder.getContext()
+                .filter(it -> it.getAuthentication() != null)
                 .map(SecurityContext::getAuthentication)
                 .filter(Authentication::isAuthenticated)
                 .map(Authentication::getPrincipal)