timers: check for undefined instead of nullish value

Author: gurgunday

lib/internal/timers.js

@@ -531,7 +531,7 @@ function getTimerCallbacks(runNextTicks) {
 
     let list;
     let ranAtLeastOneList = false;
-    while ((list = timerListQueue.peek()) != null) {
+    while ((list = timerListQueue.peek()) !== undefined) {
       if (list.expiry > now) {
         nextExpiry = list.expiry;
         return timeoutInfo[0] > 0 ? nextExpiry : -nextExpiry;