@Nullable
    @Contract(value = "null -> null; !null -> !null", pure = true)
    public static TimerTask unwrap(@Nullable TimerTask timerTask) {
        if (!(timerTask instanceof TtlTimerTask)) return timerTask;
        else return ((TtlTimerTask) timerTask).getTimerTask();
    }
Unwrap {@link TtlTimerTask} to the original/underneath one.
<p>
this method is {@code null}-safe, when input {@code TimerTask} parameter is {@code null}, return {@code null};
if input {@code TimerTask} parameter is not a {@link TtlTimerTask} just return input {@code TimerTask}.

@see #get(TimerTask)
ttl-core/src/main/java/com/alibaba/ttl3/TtlTimerTask.java