inspector: fix inspector open when there are sessions

[theanarkh]

fix inspector.open when there are sessions.

const { Session, open } = require('inspector');
const session = new Session();
session.connect();
open();

this code above will throw an error ERR_INSPECTOR_ALREADY_ACTIVATED, but the following code works.

const { Session, open } = require('inspector');
const session = new Session();
open();
session.connect();

i think we should only throw ERR_INSPECTOR_ALREADY_ACTIVATED when there is a inspector thread 🤔.

Refs: #33015
cc @joyeecheung

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines