test: http outgoing _headers setter null

Co-authored-by: Qingyu Deng <i@ayase-lab.com>

PR-URL: https://github.com/nodejs/node/pull/38881
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-http-outgoing-internal-headers.js

@@ -31,3 +31,14 @@ common.expectWarning('DeprecationWarning', warn, 'DEP0066');
       origin: ['Origin', 'localhost']
     }));
 }
+
+{
+  // Tests for _headers set method `null`
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage._headers = null;
+
+  assert.strictEqual(
+    outgoingMessage[kOutHeaders],
+    null
+  );
+}