ldap: actually fix anonymous binds (Fixes #234)

prosody/rootfs/defaults/saslauthd.conf

@@ -1,7 +1,7 @@
 {{ if eq (.Env.AUTH_TYPE | default "internal") "ldap" }}
 ldap_servers: {{ .Env.LDAP_URL }}
 ldap_search_base: {{ .Env.LDAP_BASE }}
-{{ if .Env.LDAP_BINDDN }}
+{{ if .Env.LDAP_BINDDN | default "" | toBool }}
 ldap_bind_dn: {{ .Env.LDAP_BINDDN }}
 ldap_bind_pw: {{ .Env.LDAP_BINDPW }}
 {{ end }}