Config parameter keystore_password should be nullable

Some SSL configuration may pass unencrypted private keys. PEMReader might accept that by assuming keyPassword to be null in that case (e.g. https://github.com/apache/cassandra/blob/f9e033f519c14596da4dc954875756a69aea4e78/src/java/org/apache/cassandra/security/PEMReader.java#L103).

Current configuration reader does not accept keystore_password parameter to be set null or empty in the cassandra.yaml.