[bugfix/email] Don't use plainAuth when no smtp username/password provided

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

Same as https://github.com/superseriousbusiness/gotosocial/commit/6783c6057c2dc3d1b27a97487cf1967d3030ba87 but with some formatting fixes.

closes https://github.com/superseriousbusiness/gotosocial/issues/3320

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• [x] I/we have read the GoToSocial contribution guidelines.
• [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• [x] I/we have not leveraged AI to create the proposed changes.
• [x] I/we have performed a self-review of added code.
• [x] I/we have written code that is legible and maintainable by others.
• [x] I/we have commented the added code, particularly in hard-to-understand areas.
• [ ] I/we have made any necessary changes to documentation.
• [ ] I/we have added tests that cover new code.
• [x] I/we have run tests and they pass locally with the changes.
• [x] I/we have run go fmt ./... and golangci-lint run.