LibWeb: Actually set empty serialized query to OptionalNone

Because the type returned by to_string is a String, _not_ an
Optional<String>, the following code:

if (serialized_query.is_empty())
    serialized_query = {};

Was achieving nothing at all! Make sure that the type is an
Optional<String> so that we're not just setting an empty string to an
empty string.

Tests/LibWeb/Text/expected/URL/search-params-with-no-query.txt

@@ -0,0 +1,2 @@
+https://www.birdoftheyear.org.nz/?
+https://www.birdoftheyear.org.nz/

Tests/LibWeb/Text/input/URL/search-params-with-no-query.html

@@ -0,0 +1,9 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let url = new URL("https://www.birdoftheyear.org.nz/?")
+        println(url.href);
+        url.searchParams.sort()
+        println(url.href);
+    });
+</script>

Userland/Libraries/LibWeb/DOMURL/URLSearchParams.cpp

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
- * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
+ * Copyright (c) 2023-2024, Shannon Booth <shannon@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -213,17 +213,18 @@ void URLSearchParams::append(String const& name, String const& value)
     update();
 }
 
+// https://url.spec.whatwg.org/#concept-urlsearchparams-update
 void URLSearchParams::update()
 {
     // 1. If query’s URL object is null, then return.
     if (!m_url)
         return;
 
     // 2. Let serializedQuery be the serialization of query’s list.
-    auto serialized_query = to_string();
+    Optional<String> serialized_query = to_string();
 
     // 3. If serializedQuery is the empty string, then set serializedQuery to null.
-    if (serialized_query.is_empty())
+    if (serialized_query == String {})
         serialized_query = {};
 
     // 4. Set query’s URL object’s URL’s query to serializedQuery.