fix(plugin-multi-tenant): not setting cookie properly on login (#14107)

When using something like oauth, the cookie needs to be set.

This is because the login happens elsewhere and the user is redirected.
When the admin panel loads, there is already a user so the `useEffect`
does not run since the user does not change like it does when logging in
without oauth.

The fix was to check if there is an intialValue and no matching cookie,
then sync the tenants which will also set the cookie. Also useful if a
user logs in and has a stale tenant cookie set.