stream: allow returning null from pipeline tail

Author: ronag

lib/internal/streams/pipeline.js

@@ -288,7 +288,9 @@ function pipelineImpl(streams, callback, opts) {
           then.call(ret,
                     (val) => {
                       value = val;
-                      pt.write(val);
+                      if (val != null) {
+                        pt.write(val);
+                      }
                       if (end) {
                         pt.end();
                       }

test/parallel/test-stream-pipeline.js

@@ -1511,3 +1511,18 @@ const tsp = require('timers/promises');
     assert.strictEqual(s.destroyed, true);
   }));
 }
+
+{
+  const s = new PassThrough({ objectMode: true });
+  pipeline(async function*() {
+    await Promise.resolve();
+    yield 'hello';
+    yield 'world';
+    yield 'world';
+  }, s, async function(source) {
+    return null;
+  }, common.mustCall((err, val) => {
+    assert.strictEqual(err, undefined);
+    assert.strictEqual(val, null);
+  }));
+}