ldap: fix anonymous binds

Before,
ldap_bind_dn: <novalue>
ldab_bind_pw: <novalue>
was inserted into the saslauthd.conf, which made anonymous binds impossible.

prosody/rootfs/defaults/saslauthd.conf

@@ -1,8 +1,10 @@
 {{ if eq (.Env.AUTH_TYPE | default "internal") "ldap" }}
 ldap_servers: {{ .Env.LDAP_URL }}
 ldap_search_base: {{ .Env.LDAP_BASE }}
+{{ if .Env.LDAP_BINDDN }}
 ldap_bind_dn: {{ .Env.LDAP_BINDDN }}
 ldap_bind_pw: {{ .Env.LDAP_BINDPW }}
+{{ end }}
 ldap_filter: {{ .Env.LDAP_FILTER | default "uid=%u" }}
 ldap_version: {{ .Env.LDAP_VERSION | default "3" }}
 ldap_auth_method: {{ .Env.LDAP_AUTH_METHOD | default "bind" }}