timers: check for nullish instead of falsy in loops

This prepares the code for the no-cond-assign ESLint rule.

PR-URL: #41614
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Trott

lib/internal/timers.js

@@ -490,7 +490,7 @@ function getTimerCallbacks(runNextTicks) {
 
     let list;
     let ranAtLeastOneList = false;
-    while (list = timerListQueue.peek()) {
+    while ((list = timerListQueue.peek()) != null) {
       if (list.expiry > now) {
         nextExpiry = list.expiry;
         return refCount > 0 ? nextExpiry : -nextExpiry;
@@ -511,7 +511,7 @@ function getTimerCallbacks(runNextTicks) {
 
     let ranAtLeastOneTimer = false;
     let timer;
-    while (timer = L.peek(list)) {
+    while ((timer = L.peek(list)) != null) {
       const diff = now - timer._idleStart;
 
       // Check if this loop iteration is too early for the next timer.