fix(examples): ensure working multi-tenant example with pg (#11501)

### What?
There were a couple issues with the implementation within the example
when using postgres.
- `ensureUniqueUsername` tenant was being extracted incorrectly, should
not constrain query unless it was present
- `ensureUniqueSlug` was querying by NaN when tenant was not present on
data or originalDoc
- `users` read access was not correctly extracting the tenant id in the
correct type depending on DB

Fixes https://github.com/payloadcms/payload/issues/11484

examples/multi-tenant/next-env.d.ts

@@ -2,4 +2,4 @@
 /// <reference types="next/image-types/global" />
 
 // NOTE: This file should not be edited
-// see https://nextjs.org/docs/app/building-your-application/configuring/typescript for more information.
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.