So Im taking my shower, and I think, self, why are we even checking the user-agent with a comment submission? Self answered: askimet?
My guess would be that the u-a is checked IF youre using askimet. However, for those that are not using askimet, a simple fix for JUST THAT ISSUE would seem to be to removing the user-agent references all together, starting about here in comment-functions.php
function check_comment($author, $email, $url, $comment, $user_ip, $user_agent, $comment_type) { ....
(more below that)
If you look at that function, I was right. its there for spam checking.
Anyway, that would satisfy that issue if you werent using askimet, and maybe even if you were. God knows what askimet does with blank u-a's
That said, if you look at that function, theres prolly room for other things there to be trouble as well. 2.0.*'s equivalent file contains a good deal more checking.