Empty password check no longer works

Magnolia JCRAuthenticationModule implements a check for empty passwords, see

https://git.magnolia-cms.com/projects/PLATFORM/repos/main.pub/browse/magnolia-jaas/src/main/java/info/magnolia/jaas/sp/jcr/JCRAuthenticationModule.java#156

This check is no longer working correctly. It checks for empty strings, but since the change to Hashed/BCrypted passwords, an empty password results in a non-empty hash string, and this check does not catch it.

The check needs to be implemented against the decrypted password.