Can’t remove password form UI after setting WPMS_SMTP_PASS in wp-config.php

Steps to reproduce:

1. Configure email authentication password in UI.
2. Add WPMS_ON and WPMS_SMTP_PASS constants to wp-config.php
3. Remove password from UI and save.

Now the plugin complains that no authentication is configured, and the plugin will behave like WP does by default.

Expected behaviour: because WPMS_SMTP_PASS is configured in wp-config.php, it should be used instead of the password setting from UI, and no warning should be shown to the user. Instead, a notice should be shown saying that WPMS_SMTP_PASS is set.