If you are too busy (or lazy) to put something in the subject line of an email that is going to make me want to read it, then I am probably too busy (or lazy) to read what you have sent to me.
I live and die by email, sending dozens a day and receiving hundreds. It is our number one way of communicating with customers and suppliers.
Some emails I read right away, some I save for when I am not quite so busy, and some I delete immediately. Leaving the subject line blank is a sure fire way to end up in the delete pile.
The subject line is your only opportunity to get my attention and it isn’t really to much to ask for you to give me a quick preview of what you have on your mind.