When updating already-existing users who do not have LDAP DIT entries causes PHP Warning

```
PHP Warning:  ldap_modify(): Modify: No such object in [WORDPRESS_ROOT]/wp-content/plugins/wp-ldap/includes/class-wp-ldap-api.php on line 189                                                        
Warning: ldap_modify(): Modify: No such object in [WORDPRESS_ROOT]/wp-content/plugins/wp-ldap/includes/class-wp-ldap-api.php on line 189
```