worker: make calling terminate always return a promise

This change makes terminate always return a promise even if kHandler
is null

lib/internal/worker.js

@@ -224,19 +224,18 @@ class Worker extends EventEmitter {
   }
 
   terminate(callback) {
+    if (this[kHandle] === null) return Promise.resolve();
+
     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;
       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

@@ -4,7 +4,7 @@ const common = require('../common');
 const { Worker } = require('worker_threads');
 
 // Test that calling worker.terminate() if kHandler is null should return an
-// empty promise that resolves to undefined
+// empty promise that resolves to undefined, even when a callback is passed
 
 const w = new Worker(`
 const { parentPort } = require('worker_threads');
@@ -18,8 +18,8 @@ process.once('beforeExit', common.mustCall(() => {
 
 w.on('exit', common.mustCall(() => {
   console.log('exit');
-  assert.strictEqual(w.terminate(() => null), undefined);
   w.terminate().then(returned => assert.strictEqual(returned, undefined));
+  w.terminate(() => null).then(returned => assert.strictEqual(returned, undefined));
 }));
 
 w.unref();