No "Are you sure you want to send this email without a subject?"

No "Are you sure you want to send this email without a subject?"

This is more of a suggestion, and not a problem, but why is there no tooltip that pops up if I try to send an email with no subject? I just accidentally did this. Thankfully it was to someone with whom I've had previous email interactions, so its not as big a deal as it could have been, but really, this should be a standard feature.