NAS-131033 / 25.04 / API test_mail.test_config_settings regression

[creatorcary]

While it is generally good practice to restore the system's state at the end of a test, in this case it was not allowed since the fields fromemail and others begin as empty strings and validation prevents mail.update from restoring fromemail to an empty string.

See this test for clarification.

Problematic validation:

middleware/src/middlewared/middlewared/plugins/mail.py

Line 167 in 2da0e2d

if not new['fromemail']:

[bugclerk]

Jira URL: https://ixsystems.atlassian.net/browse/NAS-131033

[bugclerk]

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