[TASK] Make admin user creation optional in CLI installer

Improve the CLI command `setup` to allow instance creation
without enforcing admin user and password to be defined.

This enables to create tiny test setups where no admin user is
needed or where users are imported from fixtures.

Resolves: #103358
Releases: main, 12.4
Change-Id: Idb5e309eb873ade7fee153a1e9b1398e4e99bfb1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83397


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benjamin Franzke <ben@bnf.dev>