Is there any use for PBKDF with empty password?
While it might be possible to use an empty password, the reason for that is most likely that any exclusion would also be known to the attacker.

However, I wouldn't call it an important test case. A PBKDF does not protect from choosing bad passwords, it only makes the dictionary attack take a lot more computational effort. If one of the first guesses is correct already, no password-based KDF will protect against that. And if the empty password is allowed, it surely is quite high in that list.

Alternatively, they could have just overlooked that case. Or they decided against it. Only the authors might be able to answer your question with certainty.