x509-cert: parse the empty string as the empty Name - #2444

RFC 4514 Section 2.1 says: "If the RDNSequence is an empty sequence, the result is the empty or zero-length string". Display does that: Name::default().to_string() is "". Section 3's grammar makes the list of RDNs optional (distinguishedName = [ relativeDistinguishedName *( COMMA relativeDistinguishedName ) ]).

RdnSequence::from_str("") split the empty string into one empty RDN and returned an error, so the empty name couldn't be read back from its own text form:

let empty = Name::default();
assert_eq!(empty.to_string(), "");
Name::from_str(&empty.to_string()); // Err on master

Empty subject names are normal in X.509: RFC 5280 4.1.2.6 allows an empty subject when subjectAltName is present.

Fix: RdnSequence::from_str returns the empty sequence for "". A missing RDN between separators (",", "CN=a,") is still an error.

from_str_empty_name in tests/name.rs fails on master and passes with this change. The x509-cert CI steps (--no-default-features, default, --features arbitrary/hazmat/sct, the feature powerset, --all-features in debug and release, on stable and 1.85), cargo fmt --check and cargo +1.90.0 clippy --all-features --tests all pass.

Found by fuzzing the RFC 4514 Display → FromStr round trip. This PR was produced by AI agents (Claude), from the fuzzing and the fix through to this description.