Mike,
The site does have a minimum length of at least one word, but that word must be seen as coming from you. What this means is that if you embed your reply within someone else's quoted text you can submit what looks like an empty reply... the same goes if you post a message with no text and only a photo. This has been the case since at least when I first joined back in 2009, if not much earlier in the site's history, so it is nothing new.
As long as you have at least a single word of text that appears to be coming from you in your post then you will not see this message.
When a member sees this message it is almost exclusively because they accidentally submitted an empty response (which is why the one word limit exists, as it does happen), submitted a post with a photo and no text, or they placed their reply within another member's quoted text when replying (which is very confusing for the next reader since your reply would look as if it were the original member's quoted text rather than your own).
HTH