The WebCrypto API specification was updated
(https://github.com/w3c/webcrypto/pull/380) to allow zero length in
deriveBits operation of HKDF and PBKDF2. This patch updates our
implementation accordingly.
Testing: Pass some WPT tests that were expected to fail.
Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>