[directory-connector] Allow anonymous LDAP bind

The directory connector requires an LDAP username and password.

However, an LDAP server can be configured to accept anonymous binds.
For example, on my LDAP server, I can run:

ldapsearch -x -H “ldap://ldap.example.com:389” -D “” -w “” -b “ou=people,dc=example,dc=com” “(uid=xxx)”