When evictableMmapped or evictable size is zero, do not throw NoSuchElementException in ShortCircuitCache#close()

/**
 * Close the cache and free all associated resources.
 */
@Override
public void close() {
  try {
    lock.lock();
    if (closed) return;
    closed = true;
    LOG.info(this + ": closing");
    maxNonMmappedEvictableLifespanMs = 0;
    maxEvictableMmapedSize = 0;
    // Close and join cacheCleaner thread.
    IOUtilsClient.cleanupWithLogger(LOG, cacheCleaner);
    // Purge all replicas.
    while (true) {
      Object eldestKey;
      try {
        eldestKey = evictable.firstKey();
      } catch (NoSuchElementException e) {
        break;
      }
      purge((ShortCircuitReplica)evictable.get(eldestKey));
    }
    while (true) {
      Object eldestKey;
      try {
        eldestKey = evictableMmapped.firstKey();
      } catch (NoSuchElementException e) {
        break;
      }
      purge((ShortCircuitReplica)evictableMmapped.get(eldestKey));
    }
  } finally {
    lock.unlock();
  }