Fix empty bytes being nullptr empty.

[GhostofCookie]

On some machines (M2 MacOS at least), the empty_byte_string is actually set as a nullptr (owned_bytes<0> results in a nullptr), which causes openssl 3 to throw an error in hkdf_extract:

error:078C0102:common libcrypto routines::passed a null parameter

Changing to owned_bytes<1> appears to fix the issue.

[bifurcation]

CI failures are unrelated.