Re: [whatwg/url] Allow encoding of null values in `application/x-www-form-urlencoded`. (Issue #732)

If we treated pairs with no key-value delimiter as having a `null` value, I think we would also have to accept that in the URL `http://example/foo&&&baz&&another`, the key-value pairs are:

```
key: "foo", value: null
key: "", value: null
key: "", value: null
key: "baz", value: null
key: "", value: null
key: "another", value: null
```

In other words, we would not be able to skip strings of empty pair delimiters (`&&&&`). AFAICT, that skipping is widely agreed upon by other URL/querystring libraries, so this would be a significant departure.

To illustrate why we would need to do that:

I have an API which allows identifying each key-value pair by its position (i.e. an index) and supports index-based operations such as inserting pairs at a particular location, replacing a region of pairs, removing a particular pair, or changing the key/value of a particular pair.

Now consider the URL `http://example/?foo&baz&another#frag`. The API allows replacing the key component of the first pair:

```
url.queryParams.replaceKey(at: 0, with: "new_key")
// result: http://example/?new_key&baz&another#frag
//                         ^^^^^^^
```

But if you replace the key with the empty string, something interesting happens:

```
url.queryParams.replaceKey(at: 0, with: "")
// result: http://example/?=&baz&another#frag
//                         ^
```

Since empty strings of delimiters are usually skipped entirely, we must insert an `=` sign in order to preserve the fact that the first pair exists and its key is the empty string.

Currently this is fine, because the presence or not of the `=` sign does not change the value component. If we started to started to say that the presence of that delimiter is meaningful, then inserting this `=` would also change the pair's value. The only way around it would be allow the following result:

```
url.queryParams.replaceKey(at: 0, with: "")
// result: http://example/?&baz&another#frag
//                         ^
```

And to say that this initial `&` is actually a pair with `(key: "", value: null)`.

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