stream: allow null as second arg in Transform callback

callback(null, null) in a Transform._transform method should be
equivalent to calling this.push(null) followed by callback(), ending
the readable side of the stream. Previously val != null blocked the
push because null == null is true in loose equality, so push(null)
was never called and the stream never emitted 'end'.

Change the guard from `val != null` to `val !== undefined` so that
null passes through to this.push(), which sets state.ended and
eventually emits 'end', matching documented behavior.

Fixes: #62769

Author: galaxy4276

lib/internal/streams/transform.js

@@ -174,7 +174,7 @@ Transform.prototype._write = function(chunk, encoding, callback) {
       return;
     }
 
-    if (val != null) {
+    if (val !== undefined) {
       this.push(val);
     }
 

test/parallel/test-stream-transform-callback-null.js

@@ -0,0 +1,66 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { Transform } = require('stream');
+
+// Passing null as the second argument to the transform callback should be
+// equivalent to calling this.push(null), signaling the end of the readable
+// side. Refs: https://github.com/nodejs/node/issues/62769
+
+{
+  // Calling callback(null, null) should end the readable side of the transform stream.
+  const t = new Transform({
+    transform(chunk, encoding, callback) {
+      callback(null, null);
+    },
+  });
+
+  t.on('end', common.mustCall());
+  t.on('data', (chunk) => {
+    // Null sentinel should not appear as a data chunk.
+    assert.fail('unexpected data event');
+  });
+
+  t.write('hello');
+  t.end();
+}
+
+{
+  // Verify callback(null, data) still works normally.
+  const t = new Transform({
+    transform(chunk, encoding, callback) {
+      callback(null, chunk);
+    },
+  });
+
+  const received = [];
+  t.on('data', (chunk) => received.push(chunk.toString()));
+  t.on('end', common.mustCall(() => {
+    assert.deepStrictEqual(received, ['hello']);
+  }));
+
+  t.write('hello');
+  t.end();
+}
+
+{
+  // Verify callback() with no second arg still works (no push).
+  const t = new Transform({
+    transform(chunk, encoding, callback) {
+      callback();
+    },
+    flush(callback) {
+      callback(null, 'flushed');
+    },
+  });
+
+  const received = [];
+  t.on('data', (chunk) => received.push(chunk.toString()));
+  t.on('end', common.mustCall(() => {
+    assert.deepStrictEqual(received, ['flushed']);
+  }));
+
+  t.write('hello');
+  t.end();
+}