[whatwg/url] What should `new URL("#")` return? (#539)

What should be the result of `new URL("#")`?
Browsers return different results and It confuses me.

In my understanding is that `new URL("#")` should throw a validation error.

1. [scheme start state](https://url.spec.whatwg.org/#scheme-start-state) 
    1. [skip] If c is an ASCII alpha, append c, lowercased, to buffer, and set state to scheme state. 
    2. [match] Otherwise, if state override is not given, set state to no scheme state and decrease pointer by 1. → go to [no schema state](https://url.spec.whatwg.org/#no-scheme-state)
2.  [no schema state](https://url.spec.whatwg.org/#no-scheme-state)
    1. [match] If base is null, or base’s cannot-be-a-base-URL flag is set and c is not U+0023 (#), validation error, return failure. → throw validation error

```js
if ( base === null || (cannot-be-a-base-URL && c !== "#")){
  throw validation error
}
```

state value

- `base` is `null` (initial value)
- cannot-be-a-base-URL flag is not set
- `c` is `#`

---

Test results:

- Firefox 80.0 → throw TypeError: URL constructor: # is not a valid URL.
- Chrome 85 → return `URL {href: "about:blank#", origin: "null", protocol: "about:", username: "", password: "", …}`
- Safari 13.1.2 → return `URL {href: "about:blank#", origin: "null", protocol: "about:", username: "", password: "", …}`
- [whatwg-url](https://github.com/jsdom/whatwg-url) → throw Error 

I can not found this test in [wpt/url](https://github.com/web-platform-tests/wpt/tree/master/url).

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