PBKDF2 derive bits should throw OperationError when length is zero

[littledivy]

WPT has a test for it https://github.com/web-platform-tests/wpt/blob/561fc4c5463673e5291894705c7a6aeb88e78d99/WebCryptoAPI/derive_bits_keys/pbkdf2.js#L118

I'm confused if the underlying implementation (following RFC 2898 Section 5.2) should already be validating that. If not so, the spec should consider adding this step.

[twiss]

Technically speaking the underlying algorithm doesn't validate, but merely requires that dkLen is a positive integer. So yeah, I agree this should be checked in this step. Thanks for raising this! If you feel like making a PR, it'd be very welcome, otherwise I can do so :)
Also, we should probably add the same check in HKDF, as well.