Do not set the salt or info if they are NULL for OpenSSL HKDF.

[vcsjones]

OpenSSL 3.x has introduced a change that stops allowing HKDF's salt to be set to NULL in openssl/openssl#27305.

This breaks our HKDF implementation which sets a NULL salt and info, depending on the mode.

This broke plenty of existing tests with failures like:

Error Message:
   Interop+Crypto+OpenSslCryptographicException : error:078C0102:common libcrypto routines::passed a null parameter
  Stack Trace:
     at Interop.Crypto.HkdfExpand(SafeEvpKdfHandle kdf, ReadOnlySpan`1 prk, String algorithm, ReadOnlySpan`1 info, Span`1 destination) in /__w/runtime-ci/runtime-ci/runtime/src/libraries/Common/src/Interop/Unix/System.Security.Cryptography.Native/Interop.EVP.Kdf.cs:line 113
   at System.Security.Cryptography.HKDF.Expand(HashAlgorithmName hashAlgorithmName, Int32 hashLength, ReadOnlySpan`1 prk, Span`1 output, ReadOnlySpan`1 info) in /__w/runtime-ci/runtime-ci/runtime/src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/HKDF.OpenSsl.cs:line 44

In this case, we no longer set the info or salt if they are NULL, and tests are green again.

This was found in a nightly run against OpenSSL 3.5. https://github.com/vcsjones/runtime-ci/actions/runs/14574054040/job/40876380622

Fixes vcsjones/runtime-ci#5

[Copilot]

Pull Request Overview

This PR updates the HKDF implementation to no longer set a NULL salt or info parameter in order to comply with OpenSSL 3.x behavior.

• Conditionally includes the salt and info parameters only when they are non-null and have a non-zero length.
• Updates the construction of the OSSL_PARAM array accordingly.

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[vcsjones]

Green run from edge CI: https://github.com/vcsjones/runtime-ci/actions/runs/14575724379/job/40881320369

[vcsjones]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/14581934220