Every once in a while, we get email sent from the site's email address to itself. No body, no subject. It's being sent by DefaultMailSystem::mail() not backdrop_mail(), apparently - we use Maillog and there's no trace of it there.
Has anyone else ever seen this? I assume it's some kind of cron symptom.
I am noticing that no one seems to recognize this problem here.
I recommend opening an issue in the core issue queue with as many details as possible. Maybe someone will experience this problem in the future and finding your report in the issue queue could be helpful to them.
I'm reluctant to do that as the only information I have is what's here. I may add some tracing code for email with an empty subject to see if I can find out more.