Description

Do not use plainAuth when no user or password.

closes #3320

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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.
[ ] 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.
[x] I/we have made any necessary changes to documentation.
[ ] I/we have added tests that cover new code.
[ ] I/we have run tests and they pass locally with the changes.
[x] I/we have run go fmt ./... and golangci-lint run.