src: minor refactor of node_trace_events.cc

Avoid unnecessary copies/extra operations & align with our style guide,
and add protection against throwing getters.

PR-URL: https://github.com/nodejs/node/pull/21867


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>