code: void ensurePrestart() {
          int wc = workerCountOf(ctl.get());
          if (wc < corePoolSize)
              addWorker(null, true);
          else if (wc == 0)
              addWorker(null, false);
      }
docs: Same as prestartCoreThread except arranges that at least one
  thread is started even if corePoolSize is 0.