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/Applications/DisplaySettings/MonitorSettingsWidget.cpp

@@ -141,7 +141,7 @@ static ErrorOr<String> display_name_from_edid(EDID::Parser const& edid)
     auto product_name = edid.display_product_name();
 
     auto build_manufacturer_product_name = [&]() -> ErrorOr<String> {
-        if (product_name.is_null() || product_name.is_empty())
+        if (product_name.is_empty())
             return TRY(String::from_deprecated_string(manufacturer_name));
         return String::formatted("{} {}", manufacturer_name, product_name);
     };

Userland/Applications/Mail/MailWidget.cpp

@@ -319,7 +319,7 @@ void MailWidget::selected_mailbox(GUI::ModelIndex const& index)
         DeprecatedString date = internal_date.to_deprecated_string();
         DeprecatedString subject = envelope.subject.value_or("(No subject)");
         if (subject.contains("=?"sv) && subject.contains("?="sv)) {
-            subject = MUST(IMAP::decode_rfc2047_encoded_words(subject));
+            subject = MUST(IMAP::decode_rfc2047_encoded_words(subject)).span();
         }
 
         StringBuilder sender_builder;
@@ -491,9 +491,9 @@ void MailWidget::selected_email_to_load(GUI::ModelIndex const& index)
     } else if (selected_alternative_encoding.equals_ignoring_ascii_case("base64"sv)) {
         auto decoded_base64 = decode_base64(encoded_data);
         if (!decoded_base64.is_error())
-            decoded_data = decoded_base64.release_value();
+            decoded_data = decoded_base64.release_value().span();
     } else if (selected_alternative_encoding.equals_ignoring_ascii_case("quoted-printable"sv)) {
-        decoded_data = IMAP::decode_quoted_printable(encoded_data).release_value_but_fixme_should_propagate_errors();
+        decoded_data = IMAP::decode_quoted_printable(encoded_data).release_value_but_fixme_should_propagate_errors().span();
     } else {
         dbgln("Mail: Unimplemented decoder for encoding: {}", selected_alternative_encoding);
         GUI::MessageBox::show(window(), DeprecatedString::formatted("The e-mail encoding '{}' is currently unsupported.", selected_alternative_encoding), "Unsupported"sv, GUI::MessageBox::Type::Information);

Userland/Applications/NetworkSettings/main.cpp

@@ -24,7 +24,7 @@ ErrorOr<int> serenity_main(Main::Arguments args)
     TRY(Core::System::unveil("/tmp/portal/window", "rw"));
     TRY(Core::System::unveil(nullptr, nullptr));
 
-    DeprecatedString adapter;
+    StringView adapter;
 
     Core::ArgsParser parser;
     parser.add_positional_argument(adapter, "Adapter to display settings for", "adapter", Core::ArgsParser::Required::No);

Userland/Applications/SoundPlayer/M3UParser.cpp

@@ -26,7 +26,7 @@ NonnullOwnPtr<M3UParser> M3UParser::from_file(StringView path)
 NonnullOwnPtr<M3UParser> M3UParser::from_memory(DeprecatedString const& m3u_contents, bool utf8)
 {
     auto parser = make<M3UParser>();
-    VERIFY(!m3u_contents.is_null() && !m3u_contents.is_empty() && !m3u_contents.is_whitespace());
+    VERIFY(!m3u_contents.is_empty() && !m3u_contents.is_whitespace());
     parser->m_m3u_raw_data = m3u_contents;
     parser->m_use_utf8 = utf8;
     return parser;

Userland/Applications/SoundPlayer/Player.cpp

@@ -40,9 +40,6 @@ Player::Player(Audio::ConnectionToServer& audio_client_connection)
 
 void Player::play_file_path(DeprecatedString const& path)
 {
-    if (path.is_null())
-        return;
-
     if (!FileSystem::exists(path)) {
         audio_load_error(path, "File does not exist"sv);
         return;

Userland/Applications/Spreadsheet/SpreadsheetView.cpp

@@ -22,7 +22,7 @@ namespace Spreadsheet {
 
 void SpreadsheetView::EditingDelegate::set_value(GUI::Variant const& value, GUI::ModelEditingDelegate::SelectionBehavior selection_behavior)
 {
-    if (value.as_string().is_null()) {
+    if (!value.is_valid()) {
         StringModelEditingDelegate::set_value("", selection_behavior);
         commit();
         return;

Userland/Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -74,7 +74,7 @@ void MemoryStatsWidget::set_graph_widget(GraphWidget& graph)
 void MemoryStatsWidget::set_graph_widget_via_name(DeprecatedString name)
 {
     m_graph_widget_name = move(name);
-    if (!m_graph_widget_name.is_null()) {
+    if (!m_graph_widget_name.is_empty()) {
         // FIXME: We assume here that the graph widget is a sibling or descendant of a sibling. This prevents more complex hierarchies.
         auto* maybe_graph = parent_widget()->find_descendant_of_type_named<GraphWidget>(m_graph_widget_name);
         if (maybe_graph) {

Userland/Applications/TextEditor/MainWidget.cpp

@@ -279,7 +279,7 @@ MainWidget::MainWidget()
 
     m_save_as_action = GUI::CommonActions::make_save_as_action([&](auto&) {
         auto extension = m_extension;
-        if (extension.is_null() && m_editor->syntax_highlighter())
+        if (extension.is_empty() && m_editor->syntax_highlighter())
             extension = Syntax::common_language_extension(m_editor->syntax_highlighter()->language());
 
         auto response = FileSystemAccessClient::Client::the().save_file(window(), m_name, extension);