Notice when saving settings with a password override

[soup-bowl]

What WordPress, PHP and Simple SMTP versions are you using?
e.g. WordPress 5.8, PHP 8.0, Simple SMTP 1.1.2 & 1.2 beta.

Describe the bug
Found in PR-43 - #43 (comment)

A notice error is generated when you save the settings page, where the password field has been over-ridden by a higher purpose.

Steps to reproduce
(with DEBUG_MODE set to true).

1. Generate a MS instance, network activate the plugin.
2. In Network Mail, set a password (tool used in this was mailtrap.io).
3. Go into a child site SMTP settings.
4. Save the settings page.

Expected behavior
The settings page saves (problem only occurs when debug mode is on).