docs: private boolean isNull(InternalCacheEntry<K, V> entry) {
        if (entry == null) {
           return true;
        } else if (entry.canExpire()) {
           if (entry.isExpired(timeService.wallClockTime())) {
              if (cacheNotifier.hasListener(CacheEntryExpired.class)) {
                 CompletionStages.join(cacheNotifier.notifyCacheEntryExpired(entry.getKey(), entry.getValue(),
                       entry.getMetadata(), ImmutableContext.INSTANCE));
              }
              return true;
           }
        }
        return false;
     }