ignore instance.actor when checking if there are local users (#13146)

* ignore `instance.actor` when checking if there are local users

We've seen this happen a few times:

* there was some AP software at $some_domain
* it gets replaced by Misskey
* before the first user can be created, an AP activity comes in
* Misskey resolves the activity
* to do this, it creates the `instance.actor` to sign its request
* now there *is* a local user, so the `meta` endpoint returns
  `requireSetup:false`
* the admin is very confused

This commit factors out the check, and doesn't count the
`instance.actor` as a real user.

* autogen bits

packages/misskey-js/src/autogen/apiClientJSDoc.ts

@@ -1,6 +1,6 @@
 /*
  * version: 2024.2.0-beta.8
- * generatedAt: 2024-01-31T01:46:47.964Z
+ * generatedAt: 2024-02-02T14:18:15.716Z
  */
 
 import type { SwitchCaseResponseType } from '../api.js';