@Nullable
    @Contract(value = "null -> null; !null -> !null", pure = true)
    @SuppressWarnings("unchecked")
    public static <T extends Executor> T unwrapTtlExecutor(@Nullable T executor) {
        if (!isTtlExecutor(executor)) return executor;

        return (T) ((ExecutorTtlWrapper) executor).unwrap();
    }
Unwrap TTL executor wrapper to the original/underneath one.
<p>
if the parameter executor is TTL wrapper, return the original/underneath executor;
otherwise, just return the input parameter executor.
<p>
NOTE: if input executor is {@code null}, return {@code null}.

@param executor input executor
@param <T>      Executor type
@see #getTtlExecutor(Executor)
@see #getTtlExecutorService(ExecutorService)
@see #getTtlScheduledExecutorService(ScheduledExecutorService)
@see #isTtlExecutor(Executor)
@see com.alibaba.ttl3.TtlWrappers#unwrap(Object)
ttl-core/src/main/java/com/alibaba/ttl3/executor/TtlExecutors.java