# fix: keep valueless query parameters when casting to a string

## Problem

A query parameter that has no value (for example `?foo` or `?foo=`) is parsed and
stored, and `has()` / `getAllQueryParameters()` report it as present, but casting
the bag (or the `Url`) back to a string silently drops it:

```php
$url = Spatie\Url\Url::fromString('https://example.com/?foo=');
$url->hasQueryParameter('foo');   // true
(string) $url;                    // "https://example.com"  <-- "foo" is gone
```

So `hasQueryParameter('foo')` is `true` while the parameter disappears from the
string representation, and the URL does not round-trip.

## Root cause

`src/QueryParameterBag.php`. `fromString()` deliberately stores valueless
parameters as `null` (added in #11, and pinned by the "empty query parameters
default to null" test):

```php
$parameters = array_map(fn ($param) => $param !== '' ? $param : null, $parameters);
```

but `__toString()` passes those `null` values straight to `http_build_query()`:

```php
return http_build_query($this->parameters, '', '&', PHP_QUERY_RFC3986);
```

`http_build_query()` skips `null` values entirely, so every valueless parameter
is removed from the output.

## The fix

Render `null` values as empty-valued parameters before building the query string,
so they survive the round-trip:

```php
$parameters = array_map(fn ($value) => $value ?? '', $this->parameters);

return http_build_query($parameters, '', '&', PHP_QUERY_RFC3986);
```

`?foo` / `?foo=` now serialize back to `foo=` (which parses to the same value),
and parameters with an actual value or an array value are unaffected.

## Tests

Added two cases to `QueryParameterBagTest`:

- `it keeps valueless parameters when being casted to a string`
  (`fromString('offset')` -> `has('offset')` true and `__toString()` === `offset=`).
- `it keeps valueless parameters alongside valued ones when being casted to a string`
  (`fromString('foo=&bar=baz')->__toString()` === `foo=&bar=baz`).

## Gate

```
composer install && vendor/bin/pest
```

(`phpunit` is intentionally blocked by this package in favour of Pest.)

Result: exit code `0` - `Tests: 110 passed`.

Counterfactual: reverting `__toString()` to pass `$this->parameters` directly to
`http_build_query()` makes both new tests fail (the valueless parameters are
dropped from the output).