http2: fix ping callback

In case there was no ack, the callback would have returned
more than the error as return value. This makes sure that is not
the case anymore.

PR-URL: https://github.com/nodejs/node/pull/20311


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>