worker: handle calling terminate when kHandler is null

This PR makes a change to the Worker.terminate() when called if the
kHandler is null. Before this pull request it was returning undefined,
but the API is expecting a promise. With the changes in this PR if
terminate is called a Promise.resolve() is returned, unless a callback
is passed in which case the old behavior stays (returns undefined).

PR-URL: #28370
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/worker.js

@@ -224,18 +224,19 @@ class Worker extends EventEmitter {
   }
 
   terminate(callback) {
-    if (this[kHandle] === null) return;
-
     debug(`[${threadId}] terminates Worker with ID ${this.threadId}`);
 
     if (typeof callback === 'function') {
       process.emitWarning(
         'Passing a callback to worker.terminate() is deprecated. ' +
         'It returns a Promise instead.',
         'DeprecationWarning', 'DEP0132');
+      if (this[kHandle] === null) return Promise.resolve();
       this.once('exit', (exitCode) => callback(null, exitCode));
     }
 
+    if (this[kHandle] === null) return Promise.resolve();
+
     this[kHandle].stopThread();
 
     // Do not use events.once() here, because the 'exit' event will always be

test/parallel/test-worker-terminate-null-handler.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { Worker } = require('worker_threads');
+
+// Test that calling worker.terminate() if kHandler is null should return an
+// empty promise that resolves to undefined, even when a callback is passed
+
+const worker = new Worker(`
+const { parentPort } = require('worker_threads');
+parentPort.postMessage({ hello: 'world' });
+`, { eval: true });
+
+process.once('beforeExit', common.mustCall(() => {
+  console.log('beforeExit');
+  worker.ref();
+}));
+
+worker.on('exit', common.mustCall(() => {
+  console.log('exit');
+  worker.terminate().then((res) => assert.strictEqual(res, undefined));
+  worker.terminate(() => null).then(
+    (res) => assert.strictEqual(res, undefined)
+  );
+}));
+
+worker.unref();