The PR#345 [1] of the WebCrypto API specification changed the type
of the deriveBits's length argument to become 'optional' and with
'null' as default value.
The affected WebCrypto algorithms (HKDF, PBKDF2, ECDH and X25519)
will be adapted to handle the case of a null length properly.
[1] https://github.com/w3c/webcrypto/pull/345
Differential Revision: https://phabricator.services.mozilla.com/D217532
83c38a9417