process: check for null instead of falsy in while loop

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/process/task_queues.js

@@ -68,7 +68,7 @@ function runNextTicks() {
 function processTicksAndRejections() {
   let tock;
   do {
-    while (tock = queue.shift()) {
+    while ((tock = queue.shift()) !== null) {
       const asyncId = tock[async_id_symbol];
       emitBefore(asyncId, tock[trigger_async_id_symbol], tock);