`ServiceInfo` with empty `alpn_http_version` is invalid

[mxinden]

For "no-default-alpn", the presentation and wire-format values MUST be empty. When "no-default-alpn" is specified in an RR, "alpn" must also be specified in order for the RR to be "self-consistent" (Section 2.4.3).

https://www.rfc-editor.org/rfc/rfc9460.html#section-7.1.1

Make this invalid state impossible through the Rust type system.

happy-eyeballs/src/lib.rs

Line 302 in 0b8095e

pub alpn_http_versions: HashSet<HttpVersion>,