stream: use null for the error argument

When no error occurs, use `null` instead of `undefined` for the `error`
argument of the `writable.write()` and `writable.end()` callbacks.

Fixes: #44290
PR-URL: #44312
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>

lib/internal/streams/writable.js

@@ -497,7 +497,7 @@ function afterWrite(stream, state, count, cb) {
 
   while (count-- > 0) {
     state.pendingcb--;
-    cb();
+    cb(null);
   }
 
   if (state.destroyed) {
@@ -640,8 +640,10 @@ Writable.prototype.end = function(chunk, encoding, cb) {
   }
 
   if (typeof cb === 'function') {
-    if (err || state.finished) {
+    if (err) {
       process.nextTick(cb, err);
+    } else if (state.finished) {
+      process.nextTick(cb, null);
     } else {
       state[kOnFinished].push(cb);
     }
@@ -742,7 +744,7 @@ function finish(stream, state) {
 
   const onfinishCallbacks = state[kOnFinished].splice(0);
   for (let i = 0; i < onfinishCallbacks.length; i++) {
-    onfinishCallbacks[i]();
+    onfinishCallbacks[i](null);
   }
 
   stream.emit('finish');

test/parallel/test-stream-writable-end-cb-error.js

@@ -36,7 +36,7 @@ const stream = require('stream');
   let called = false;
   writable.end('asd', common.mustCall((err) => {
     called = true;
-    assert.strictEqual(err, undefined);
+    assert.strictEqual(err, null);
   }));
 
   writable.on('error', common.mustCall((err) => {

test/parallel/test-stream2-writable.js

@@ -194,7 +194,8 @@ for (let i = 0; i < chunks.length; i++) {
 {
   // Verify write callbacks
   const callbacks = chunks.map(function(chunk, i) {
-    return [i, function() {
+    return [i, function(err) {
+      assert.strictEqual(err, null);
       callbacks._called[i] = chunk;
     }];
   }).reduce(function(set, x) {
@@ -225,15 +226,19 @@ for (let i = 0; i < chunks.length; i++) {
 {
   // Verify end() callback
   const tw = new TestWriter();
-  tw.end(common.mustCall());
+  tw.end(common.mustCall(function(err) {
+    assert.strictEqual(err, null);
+  }));
 }
 
 const helloWorldBuffer = Buffer.from('hello world');
 
 {
   // Verify end() callback with chunk
   const tw = new TestWriter();
-  tw.end(helloWorldBuffer, common.mustCall());
+  tw.end(helloWorldBuffer, common.mustCall(function(err) {
+    assert.strictEqual(err, null);
+  }));
 }
 
 {