Uri does not parse valid URIs with empty authority portion

[dfawley]

Per RFC 3986, a valid URI may contain an empty authority (via an empty reg-name). However, it does not parse correctly by the Uri type:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=54db213feefae8e5b47294b4fcdbdad7

[src/main.rs:6:5] "file:///a/b/c".parse::<Uri>() = Err(
    InvalidUri(
        InvalidFormat,
    ),

Similarly, from_parts complains specifically that the authority is missing:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0d31ed893d7f6cb8b37df6353eecfe8a

[src/main.rs:11:5] Uri::from_parts(parts) = Err(
    InvalidUriParts(
        InvalidUri(
            AuthorityMissing,
        ),
    ),

I could look into a change here, but note this fix would also be a breaking behavior change, and if any users are intentionally using this to enforce a non-empty authority, then they would be impacted.

cc @seanmonstar @LucioFranco - thanks!