AK+Everywhere: Remove the null state of DeprecatedString

This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>

Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.

Userland/Services/LaunchServer/Launcher.cpp

@@ -188,7 +188,7 @@ Optional<StringView> Launcher::mime_type_for_file(DeprecatedString path)
 
 bool Launcher::open_url(const URL& url, DeprecatedString const& handler_name)
 {
-    if (!handler_name.is_null())
+    if (!handler_name.is_empty())
         return open_with_handler_name(url, handler_name);
 
     if (url.scheme() == "file")

Userland/Services/SystemServer/Service.cpp

@@ -156,8 +156,8 @@ ErrorOr<void> Service::spawn(int socket_fd)
 
     if (pid == 0) {
         // We are the child.
-        if (!m_working_directory.is_null())
-            TRY(Core::System::chdir(m_working_directory));
+        if (m_working_directory.has_value())
+            TRY(Core::System::chdir(*m_working_directory));
 
         struct sched_param p;
         p.sched_priority = m_priority;
@@ -167,9 +167,9 @@ ErrorOr<void> Service::spawn(int socket_fd)
             VERIFY_NOT_REACHED();
         }
 
-        if (!m_stdio_file_path.is_null()) {
+        if (m_stdio_file_path.has_value()) {
             close(STDIN_FILENO);
-            auto const fd = TRY(Core::System::open(m_stdio_file_path, O_RDWR, 0));
+            auto const fd = TRY(Core::System::open(*m_stdio_file_path, O_RDWR, 0));
             VERIFY(fd == 0);
 
             dup2(STDIN_FILENO, STDOUT_FILENO);
@@ -288,13 +288,13 @@ Service::Service(Core::ConfigFile const& config, StringView name)
 
     set_name(name);
     m_executable_path = config.read_entry(name, "Executable", DeprecatedString::formatted("/bin/{}", this->name()));
-    m_extra_arguments = config.read_entry(name, "Arguments", "");
-    m_stdio_file_path = config.read_entry(name, "StdIO");
+    m_extra_arguments = config.read_entry(name, "Arguments");
+    m_stdio_file_path = config.read_entry_optional(name, "StdIO");
 
-    DeprecatedString prio = config.read_entry(name, "Priority");
+    auto prio = config.read_entry_optional(name, "Priority");
     if (prio == "low")
         m_priority = 10;
-    else if (prio == "normal" || prio.is_null())
+    else if (prio == "normal" || !prio.has_value())
         m_priority = 30;
     else if (prio == "high")
         m_priority = 50;
@@ -304,9 +304,9 @@ Service::Service(Core::ConfigFile const& config, StringView name)
     m_keep_alive = config.read_bool_entry(name, "KeepAlive");
     m_lazy = config.read_bool_entry(name, "Lazy");
 
-    m_user = config.read_entry(name, "User");
-    if (!m_user.is_null()) {
-        auto result = Core::Account::from_name(m_user, Core::Account::Read::PasswdOnly);
+    m_user = config.read_entry_optional(name, "User");
+    if (m_user.has_value()) {
+        auto result = Core::Account::from_name(*m_user, Core::Account::Read::PasswdOnly);
         if (result.is_error()) {
             warnln("Failed to resolve user {}: {}", m_user, result.error());
         } else {
@@ -315,7 +315,7 @@ Service::Service(Core::ConfigFile const& config, StringView name)
         }
     }
 
-    m_working_directory = config.read_entry(name, "WorkingDirectory");
+    m_working_directory = config.read_entry_optional(name, "WorkingDirectory");
     m_environment = config.read_entry(name, "Environment");
     m_system_modes = config.read_entry(name, "SystemModes", "graphical").split(',');
     m_multi_instance = config.read_bool_entry(name, "MultiInstance");
@@ -324,7 +324,7 @@ Service::Service(Core::ConfigFile const& config, StringView name)
     DeprecatedString socket_entry = config.read_entry(name, "Socket");
     DeprecatedString socket_permissions_entry = config.read_entry(name, "SocketPermissions", "0600");
 
-    if (!socket_entry.is_null()) {
+    if (!socket_entry.is_empty()) {
         Vector<DeprecatedString> socket_paths = socket_entry.split(',');
         Vector<DeprecatedString> socket_perms = socket_permissions_entry.split(',');
         m_sockets.ensure_capacity(socket_paths.size());

Userland/Services/SystemServer/Service.h

@@ -54,7 +54,7 @@ private:
     // Extra arguments, starting from argv[1], to pass when exec'ing.
     DeprecatedString m_extra_arguments;
     // File path to open as stdio fds.
-    DeprecatedString m_stdio_file_path;
+    Optional<DeprecatedString> m_stdio_file_path;
     int m_priority { 1 };
     // Whether we should re-launch it if it exits.
     bool m_keep_alive { false };
@@ -65,9 +65,9 @@ private:
     // Whether we should only spawn this service once somebody connects to the socket.
     bool m_lazy;
     // The name of the user we should run this service as.
-    DeprecatedString m_user;
+    Optional<DeprecatedString> m_user;
     // The working directory in which to spawn the service.
-    DeprecatedString m_working_directory;
+    Optional<DeprecatedString> m_working_directory;
     // System modes in which to run this service. By default, this is the graphical mode.
     Vector<DeprecatedString> m_system_modes;
     // Whether several instances of this service can run at once.

Userland/Services/Taskbar/QuickLaunchWidget.cpp

@@ -169,7 +169,7 @@ static DeprecatedString sanitize_entry_name(DeprecatedString const& name)
 ErrorOr<void> QuickLaunchWidget::add_or_adjust_button(DeprecatedString const& button_name, NonnullOwnPtr<QuickLaunchEntry>&& entry)
 {
     auto file_name_to_watch = entry->file_name_to_watch();
-    if (!file_name_to_watch.is_null()) {
+    if (!file_name_to_watch.is_empty()) {
         if (!m_watcher) {
             m_watcher = TRY(Core::FileWatcher::create());
             m_watcher->on_change = [this](Core::FileWatcherEvent const& event) {

Userland/Services/WindowServer/ScreenLayout.ipp

@@ -31,7 +31,7 @@ bool ScreenLayout::is_valid(DeprecatedString* error_msg) const
     int smallest_y = 0;
     for (size_t i = 0; i < screens.size(); i++) {
         auto& screen = screens[i];
-        if (screen.mode == Screen::Mode::Device && (screen.device->is_empty() || screen.device->is_null())) {
+        if (screen.mode == Screen::Mode::Device && screen.device->is_empty()) {
             if (error_msg)
                 *error_msg = DeprecatedString::formatted("Screen #{} has no path", i);
             return false;