A registered letter ensures that once someone signed it off, they cannot claim that they are not aware of the content (generally speaking).
Why someone would not bring to court the empty envelope claiming that this is what they received?
This is easily countered by sending a letter which has the address directly printed on it and accordingly folded.
One of the possible limitations is the number of pages (it works best with one) but I can imagine that one can put more pages and it is less credible to claim "we just received page one" than "the envelope was empty".
But even limiting the question to one page and since the act of providing paperwork to the adversary is so important in legal proceedings (and often requires registered letters, at least in France and generally EU) I am wondering why empty envelopes (and how to counter them) is not an issue.