[BUGFIX] Exclude empty passwords from password hashing check

The test for insecure passwords handles empty passwords as insecure and
recommends to use the saltedpasswords scheduler task to convert all
insecure passwords. But the scheduler task doesn't convert empty
passwords, so a never ending story exists. Therefore this patch exludes
empty passwords being handled as insecure.

Resolves: #36244
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I6bdd5c8807a07841850c1e4c22afe57eec28b335
Reviewed-on: https://review.typo3.org/23432
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind