I don't know when this started, but WordPress auto-generates robots.txt if you don't have one. In my opinion, this is a terrible change. In my case, the auto-generated WordPress robots.txt file is sending robots to my admin page (?!) and this ended up getting indexed in DuckDuckGo, very strange.
WordPress created some kind of "filter" to configure robots.txt behavior, but a) I can't find any code to keep WordPress out of robots.txt entirely, and b) even if I manage to code something, that only affects one theme and one blog at a time.
I'd say this is a flaw in WordPress, assuming we all want an auto-generated robots.txt file.
Creating a "blank" robots.txt forces WordPress to stay out of my business, and sends the appropriate message to robots.
In short, if you see a blank robots.txt used for a WordPress blog (or some other software) this could be the reason: to prevent the auto-generation of something worse than a blank file.