HKDF in cryptography before 1.5.2 returns an empty byte-string if used with a length less than algorithm.digest_size.
https://github.com/pyca/cryptography/issues/3211
https://github.com/pyca/cryptography/commit/b924696b2e8731f39696584d12cceeb3aeb2d874
https://cryptography.io/en/latest/changelog