http2: use `Object.create(null)` for `getHeaders`

refactor `getHeaders` to initialize headers using
`Object.create(null)`

Refs: https://github.com/nodejs/node/issues/29829

PR-URL: https://github.com/nodejs/node/pull/33188


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sebastiaan Deckers <sebdeckers83@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>