DS-1781 : Ensure LDAP "email_field" is not ignored entirely

Simple PR to ensure the LDAP "email_field" setting is no longer entirely ignored. Previously, the "if" logic in the code was incorrect (as described in DS-1781)

https://jira.duraspace.org/browse/DS-1781

This also logs a warning if during auto-registration the email cannot be determined (as LDAPAuthentication plugin sets email = netid in this scenario). In many use cases, if the "email" cannot be determined, it is likely a misconfiguration of the LDAP plugin (hence the warning).