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 (mostly) a cherry-pick of 95ede12 from upstream. I've removed
the bit in ldap_test.go, which is unrelated to the security issue.

This fixes CVE-2017-14623.

https://github.com/go-ldap/ldap/commit/95ede1266b237bf8e9aa5dce0b3250e51bfefe66

Closes: #876404