- Don't create the mail queue when the mailer is disabled to prevent
`SendMail()` from blocking indefinitely
- Add guard clause in `SendMail()` to return early when mailer is
disabled or queue is nil
- Add test to verify notifications don't block when mailer is disabled
This implements the changes from #1080 with the review feedback
addressed (using `package notifications` instead of `package
notifications_test`).
Closes#1080
8987f0890a