fix: keep valueless query parameters when casting to a string

Valueless query parameters (for example ?foo) are stored as null, and has()
reports them as present, but __toString() passed those null values to
http_build_query() which skips them entirely, dropping the parameters from the
output. Render null values as empty-valued parameters so they survive a
round-trip.

src/QueryParameterBag.php

@@ -65,6 +65,10 @@ class QueryParameterBag implements \Stringable
 
     public function __toString(): string
     {
-        return http_build_query($this->parameters, '', '&', PHP_QUERY_RFC3986);
+        // Valueless parameters are stored as null, which http_build_query() skips;
+        // render them with an empty value so they survive.
+        $parameters = array_map(fn ($value) => $value ?? '', $this->parameters);
+
+        return http_build_query($parameters, '', '&', PHP_QUERY_RFC3986);
     }
 }