qs.stringify with allowEmptyArrays does not encode the keys of empty arrays, allowing query string parameter injection

### Summary

`qs.stringify` with `allowEmptyArrays: true` writes the key of an empty array without encoding it, even though keys are encoded by default. A key that contains `&` (or the configured `delimiter`), `=`, or `#` therefore becomes query string syntax instead of data, so a caller-supplied key name can add parameters to the serialized query string, or cut it short.

### Details

In `lib/stringify.js`, the `allowEmptyArrays` branch returns `adjustedPrefix + '[]'` early, before the code that passes every other key through `encoder` and `formatter`. So `{ "x&k=v": ["1"] }` serializes as `x%26k%3Dv%5B0%5D=1`, but `{ "x&k=v": [] }` serializes as `x&k=v[]`. The early return applies at any depth (`{ "a": { "b&k=v": [] } }` gives `a[b&k=v][]`), under every `arrayFormat`, with `allowDots`, and with a custom `encoder`, which is never called for that key. With `encode: false` or `encodeValuesOnly: true`, keys are intentionally left unencoded on every path; those configurations are not affected.

#### PoC

```js

var qs = require("qs");

var query = qs.stringify({ q: "hi", "x&apiKey=evil&y": [] }, { allowEmptyArrays: true });

// "q=hi&x&apiKey=evil&y[]"

qs.parse(query);

// { q: "hi", x: "", apiKey: "evil", y: [""] }

qs.stringify({ "x#": [], z: "1" }, { allowEmptyArrays: true });

// "x#[]&z=1": in a URL, everything after "#" becomes the fragment

```

#### Fix

Applied in 234d505 on `main` and released as v6.17.0, and backported to v6.16.2, v6.15.4, v6.14.3, v6.13.4, and v6.12.6: the key of an empty array goes through `encoder` and `formatter` exactly as the key of any other value does, so `{ "x&apiKey=evil&y": [] }` now serializes as `x%26apiKey%3Devil%26y[]`. Keys that need no encoding serialize as before, with two exceptions that match every other path: the brackets of a nested key are now encoded (`a[b][]` becomes `a%5Bb%5D[]`, which `qs.parse` reads identically), and with `encodeDotInKeys`, a dot in the key becomes `%252E` instead of `%2E` (which `decodeDotInKeys` previously misread as nesting).

### Affected versions

- `>=6.12.0 <6.12.6`, fixed in v6.12.6

- `>=6.13.0 <6.13.4`, fixed in v6.13.4

- `>=6.14.0 <6.14.3`, fixed in v6.14.3

- `>=6.15.0 <6.15.4`, fixed in v6.15.4

- `>=6.16.0 <6.16.2`, fixed in v6.16.2

v6.17.0 and later are not affected.

`allowEmptyArrays` was introduced in f22b2bc and first shipped in v6.12.0, with this early return. Earlier versions have no `allowEmptyArrays` option and are not affected.

### Impact

An application that passes an object whose key names come from untrusted input to `qs.stringify` with `allowEmptyArrays: true` (for example, to forward a parsed JSON body or user-selected fields to an upstream service) can have parameters injected into the resulting query string. The injected parameters are not keys of the object, so deleting or overwriting a key on the object (such as setting a trusted `apiKey` after copying the user's fields) does not remove them; whether the attacker's value or the application's wins depends on how the receiving side handles a duplicate parameter. A `#` in such a key ends the query string early when the result is placed in a URL. `allowEmptyArrays` is off by default, and only keys whose value (after any `filter`) is an empty array are affected.