pkcs5: allow empty algId prf parameter for PBES2 params

[ancwrd1]

Rationale: some applications and libraries produce PBES2/PBKDF2 structures without the terminating NULL value specified in the prf algorithm field.
This PR relaxes the decoding requirements.

According to RFC4231 the NULL parameter should be present but is not an absolute requirement.

See for example this issue: ancwrd1/p12-keystore#2

[tarcieri]

some applications and libraries produce PBES2/PBKDF2 structures without the terminating NULL value specified in the prf algorithm field.

Which ones? Can you include a reference as to how the test vector you're providing was generated, at least in the comments?

[ancwrd1]

Specifically, this: https://github.com/smallstep/cli
It produces PKCS#12 file without the algorithm parameter value. It works fine with e.g. openssl or Java KeyStore API, but fails with my p12-keystore crate which is based on RustCrypto.

I have added a note in the comments.