AK: Use a null value for Optional<(Fly)String>s empty value

Akin to
LadybirdBrowser/ladybird@2457118

Author: Hendiadyoin1

Diff for: AK/FlyString.h

@@ -15,6 +15,9 @@
 
 namespace AK {
 
+template<>
+struct Traits<FlyString>;
+
 class FlyString {
     AK_MAKE_DEFAULT_MOVABLE(FlyString);
     AK_MAKE_DEFAULT_COPYABLE(FlyString);
@@ -79,6 +82,12 @@ class FlyString {
         return (... || this->operator==(forward<Ts>(strings)));
     }
 
+    FlyString(Badge<Optional<FlyString>>)
+        : m_data(nullptr)
+    {
+    }
+    ALWAYS_INLINE constexpr bool is_null(Badge<Traits<FlyString>> badge) { return m_data.is_null(move(badge)); }
+
 private:
     explicit FlyString(Detail::StringBase data)
         : m_data(move(data))
@@ -91,6 +100,9 @@ class FlyString {
 template<>
 struct Traits<FlyString> : public DefaultTraits<FlyString> {
     static unsigned hash(FlyString const&);
+
+    constexpr static auto special_optional_empty_value(Badge<Optional<FlyString>> badge) { return FlyString(move(badge)); }
+    constexpr static bool optional_has_value(String const& str) { return !str.is_null(Badge<Traits<FlyString>> {}); }
 };
 
 template<>

Diff for: AK/String.h

@@ -202,6 +202,11 @@ class String : public Detail::StringBase {
     requires(IsSame<RemoveCVReference<T>, StringView>)
     static ErrorOr<String> from_byte_string(T&&) = delete;
 
+    constexpr String(Badge<Optional<String>>)
+        : StringBase(nullptr)
+    {
+    }
+
 private:
     friend class ::AK::FlyString;
 
@@ -216,6 +221,9 @@ class String : public Detail::StringBase {
 template<>
 struct Traits<String> : public DefaultTraits<String> {
     static unsigned hash(String const&);
+
+    constexpr static auto special_optional_empty_value(Badge<Optional<String>> badge) { return String(move(badge)); }
+    constexpr static bool optional_has_value(String const& str) { return !str.is_null(Badge<Traits<String>> {}); }
 };
 
 template<>

Diff for: AK/StringBase.cpp

@@ -117,7 +117,7 @@ ErrorOr<StringBase> StringBase::substring_from_byte_offset_with_shared_superstri
 
 void StringBase::destroy_string()
 {
-    if (!is_short_string())
+    if (!is_short_string() && m_raw != 0)
         m_data->unref();
 }
 

Diff for: AK/StringBase.h

@@ -69,9 +69,17 @@ class StringBase {
 
     [[nodiscard]] bool operator==(StringBase const&) const;
 
-    [[nodiscard]] ALWAYS_INLINE FlatPtr raw(Badge<FlyString>) const { return bit_cast<FlatPtr>(m_data); }
+    [[nodiscard]] ALWAYS_INLINE FlatPtr raw(Badge<FlyString>) const { return m_raw; }
+    template<OneOf<String, FlyString> T>
+    [[nodiscard]] ALWAYS_INLINE constexpr bool is_null(Badge<Traits<T>>) const { return m_raw; }
 
 protected:
+    // For Optional
+    constexpr explicit StringBase(nullptr_t)
+        : m_raw { 0 }
+    {
+    }
+
     template<typename Func>
     ErrorOr<void> replace_with_new_string(size_t byte_count, Func&& callback)
     {
@@ -124,6 +132,7 @@ class StringBase {
     union {
         ShortString m_short_string;
         Detail::StringData const* m_data { nullptr };
+        FlatPtr m_raw;
     };
 };