[TASK] Prevent password policy warning in BackendUserPasswordCheck

The dataHandler hook implementation `BackendUserPasswordCheck` sets
a random password for a new backend user, if the provided password
is empty in `$incomingFieldArray`. The generated password is a
random HMAC string, which does not fulfill the requirements of
the default TYPO3 password policy. As a result, a password policy
warning is shown.

This change ensures, that the generated random password at least
fulfills the default TYPO3 password policy.

Resolves: #103138
Releases: main, 12.4
Change-Id: I20ef7c5958d539533cc76ca41c4cc7b5ab07e594
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82986


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>