OPENDJ-3095 Upgrade: uninstallation of replicated server throws a NPE

The problem happened because the connection is defaulted to the adminUID, but the password is null.

3.0.0 code handled this gracefully although it was reporting an error while running uninstall.
3.5.0 code fails with a NPE on the call to password.toCharArray().

The fix consists in propagating differently the bind DN/password information
and in treating null bindDN or null password to be an anonymous bind.