update

test/output-utils.test.js

@@ -24,7 +24,7 @@ test('process format emits message with changed fields only', () => {
   assert.deepEqual(first.payload, { a: 1, b: 2 });
 
   const second = out.formatMsg({ a: 1, b: 2 }, config, 'process');
-  assert.equal(second, undefined);
+  assert.equal(second, null);
 
   const third = out.formatMsg({ a: 1, b: 3, c: { x: 1 } }, config, 'process');
   assert.deepEqual(third.payload, { b: 3, c: JSON.stringify({ x: 1 }) });