crypto: pass empty passphrases to OpenSSL properly

This solves two related problems:

1. PrivateKeyEncodingConfig was unable to distinguish between "no passphrase" and zero-length passphrases, since both may be stored as ByteSource(nullptr, 0).
2. OpenSSL uses its default key callback when a cipher was specified, but a nullptr for the passphrase. However, this did happen when an empty passphrase was specified, because malloc(0) is allowed to return a nullptr.

I'm not sure why these problems don't affect older versions.

Fixes: https://github.com/nodejs/node/issues/35898

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines