Require explicit intention for empty password. This is normally used for unauthenticated bind, and https://tools.ietf.org/html/rfc4513#section-5.1.2 recommends:
> Clients SHOULD disallow an empty password input to a Name/Password > Authentication user interface
This is a cherry-pick of 95ede12 from upstream, which fixes CVE-2017-14623.
