[whatwg/url] Verify domain is not empty after "domain to ASCII" (#497)

The empty host is not allowed in special non-"file" URLs, but
`domain to ASCII` result can be the empty string, even if the
input is not empty, for example:

* the input consists entirely of IDNA ignored code points;
* the input is `xn--`.

The `domain to ASCII` do not fail when it is called from `host parser`,
because `VerifyDnsLength` is false.
So we need additional check after it.

An interesting case is with file URLS, as empty host is allowed. But most
browsers report failure as well. I tested URLs with the `U+00AD`
percent encoded in host:

| URL               | Chrome    | Firefox     | Safari    |
|-------------------|-----------|-------------|-----------|
| `file://%C2%AD/p` | TypeError | `file:///p` | TypeError |
| `http://%C2%AD/p` | TypeError | TypeError   | TypeError |

Note. To find more IDNA ignored code points refer to IDNA Mapping Table:
https://unicode.org/reports/tr46/#IDNA_Mapping_Table


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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/url/pull/497


-- Commit Summary --

  * Verify domain is not empty after "domain to ASCII"

-- File Changes --

    M url.bs (2)

-- Patch Links --

https://github.com/whatwg/url/pull/497.patch

https://github.com/whatwg/url/pull/497.diff


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