refactor(settings): Use parameters explicitly when setting mail settings

[provokateurin]

Summary

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[joshtrichards]

1) OCA\Settings\Tests\Controller\MailSettingsControllerTest::testSetMailSettings
TypeError: OCA\Settings\Controller\MailSettingsController::setMailSettings(): Argument #8 ($mail_sendmailmode) must be of type string, null given, called in /home/runner/actions-runner/_work/server/server/apps/settings/tests/Controller/MailSettingsControllerTest.php on line 90

I guess mail_sendmailmode needs to be nullable (or the tests updated to pass '' perhaps?)

[provokateurin]

I guess mail_sendmailmode needs to be nullable (or the tests updated to pass '' perhaps?)

Null is not allowed (or at least never sent by the frontend), so the test just needs to use ''.