Dealing with students who sent email with empty body but the homework attached

I don't really think this is odd: they have been asked to send the assignment by email and they did. There isn't really a need to say anything further and they didn't. Maybe it would be slightly more courteous if they were to add a couple of words of greeting, but your job is not to be Emily Post.

If the assignments were submitted on paper, and they left their assignment in your mailbox, would you insist that they include a note wishing you a nice day? No, that would be silly.

There isn't anything you need to do about this.

When teaching large classes or multiple classes, it can be very helpful if the email, or even better the subject line, contains the key information about the class, section, group and assignment (and possibly TA). It should be the responsibility of the instructor to tell the students what is expected, if anything, in the syllabus and on the assignment itself. Those instructions should make penalties, if any, clear. In the absence of instructions, it is not worth saying anything.

I would be hesitant about imposing penalties mid term, but asking students to include additional information seems reasonable. Just make what you want clear.

Unnecessary courtesies (like greetings) and stating the obvious (like saying that attachment is attached) only waste YOUR time and increase YOUR workload. This might not be obvious at the first sight, but if they all did same that would save YOU considerable time in total. In some company environments this is standard practice, and adding meaningless greetings is what's frowned upon (between close coworkers, like you and your students).