An empty authority with an unknown spec is not parsed properly

[llimllib]

This issue comes from Bryan Nebold's post about DID URLs, and I think trurl is parsing empty authorities incorrectly.

The atmosphere URI at://did:plc:whatever/more/path/here is invalid, because did:plc:whatever is an invalid authority; it does not conform to the host:port syntax from RFC 3986. trurl is well within its rights to refuse to parse it:

$ trurl --json 'at://did:plc:vwzwgnygau7ed7b7wt5ux7y2/app.bsky.feed.post/3k5nobkf2w72g' 
trurl note: Port number was not a decimal number between 0 and 65535 [at://did:plc:vwzwgnygau7ed7b7wt5ux7y2/app.bsky.feed.post/3k5nobkf2w72g]
[]

However, the URI at:///did:plc:whatever/more/path/here has an extra slash, which should indicate an empty authority, and did:plc:whatever/more/path/here ought to be the path. Firefox parses it like this:

However, trurl treats it exactly the same, tries to parse a port out of what ought to be the first segment of the path, and exits with an error:

$ trurl --json 'at:///did:plc:vwzwgnygau7ed7b7wt5ux7y2/app.bsky.feed.post/3k5nobkf2w72g'
trurl note: Port number was not a decimal number between 0 and 65535 [at:///did:plc:vwzwgnygau7ed7b7wt5ux7y2/app.bsky.feed.post/3k5nobkf2w72g]
[]

If we simplify the URL to see what's going on, trurl will tell us it has parsed the path's first segment as a host:

$ trurl --json 'at:///foo'
[
  {
    "url": "at://foo/",
    "parts": {
      "scheme": "at",
      "host": "foo",
      "path": "/"
    }
  }
]

trurl appears to be silently removing a semantically important front-slash.

For whatever it's worth, curl seems to behave identically, parsing foo as the host:

$ curl -w '%{url.scheme} %{url.host} %{url.port} %{url.path}\n' 'at:///foo'
curl: (1) Protocol "at" not supported
at foo  /