fix: SMTP connection resource validation in Email class destructor

[michalsn]

Description
This PR fixes a bug where the destructor was checking $this->SMTPConnect !== null to validate SMTP connections.

However, fclose() creates closed resources that are not null, which led to unnecessary cleanup attempts on already-closed connections.

When the SMTPKeepAlive option is false (the default), the quit command is called before the destructor is reached.

Fixes#9647

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

No tests are provided, as I'm not sure how this could be tested effectively.

However, since this is a regression from v4.6.1, I believe it's acceptable without tests in this case.

[samsonasik]

Thank you @michalsn