How automatically block emails with an address but a blank message?

Why is the email empty?
One possibility is that an anti-spam filter somewhere stripped the content from it, apparently.
Another is that it's a method used by spammers to determine live email addresses.

Some email servers provide anti-spam features- so try looking there on your server where your account is hosted, assuming you only get these messages via one account or server.

Otherwise you would need to set up a mail rule and the means to do it e.g.
Blocking emails with blank subject line and blank message body

If neither your server nor MS365 (Outlook?) offers such, then consider investing in and using an appropriate 3rd party anti-spam program.

I don't use Outlook so I can't comment on that, sorry.
Your issue was avoided/not addressed in this thread, although a suggestion was made as to how to ask for help:
https://answers.microsoft.com/en-us/...a1bfb1b?page=1

You have options available under the Junk Mail tab to restrict incoming mail. Pop over and see what suits you the best.

For the latter, if not supported by Office, as I said, you will need an appropriate 3rd party program. The way these work is that you link them to each account you wish to protect so that when receiving mail the 3rd party program is automatically used to filter mail.

Mailwasher is an example - I don't know if it provides appropriate rules. I use Spamihilator (free) in conjunction with Thunderbird (and also with a second small mail handling program, Poppeeper which I use to periodically scan for mail on a kind of pre-processing basis before receiving remaining mail for filing with Tbird)- but I know that doesn't provide appropriate rules for this case.