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/Libraries/LibWeb/CSS/CSSNamespaceRule.cpp

@@ -40,7 +40,7 @@ DeprecatedString CSSNamespaceRule::serialized() const
     builder.append("@namespace "sv);
 
     // followed by the serialization as an identifier of the prefix attribute (if any),
-    if (!m_prefix.is_empty() && !m_prefix.is_null()) {
+    if (!m_prefix.is_empty()) {
         serialize_an_identifier(builder, m_prefix);
         // followed by a single SPACE (U+0020) if there is a prefix,
         builder.append(" "sv);

Userland/Libraries/LibWeb/CSS/CSSStyleRule.cpp

@@ -53,23 +53,23 @@ DeprecatedString CSSStyleRule::serialized() const
     builder.append(" {"sv);
 
     // 2. Let decls be the result of performing serialize a CSS declaration block on the rule’s associated declarations, or null if there are no such declarations.
-    auto decls = declaration().serialized();
+    auto decls = declaration().length() > 0 ? declaration().serialized() : Optional<DeprecatedString>();
 
     // FIXME: 3. Let rules be the result of performing serialize a CSS rule on each rule in the rule’s cssRules list, or null if there are no such rules.
-    DeprecatedString rules;
+    Optional<DeprecatedString> rules;
 
     // 4. If decls and rules are both null, append " }" to s (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)) and return s.
-    if (decls.is_null() && rules.is_null()) {
+    if (!decls.has_value() && !rules.has_value()) {
         builder.append(" }"sv);
         return builder.to_deprecated_string();
     }
 
     // 5. If rules is null:
-    if (rules.is_null()) {
+    if (!rules.has_value()) {
         //    1. Append a single SPACE (U+0020) to s
         builder.append(' ');
         //    2. Append decls to s
-        builder.append(decls);
+        builder.append(*decls);
         //    3. Append " }" to s (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)).
         builder.append(" }"sv);
         //    4. Return s.

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -36,9 +36,9 @@ static inline bool matches_lang_pseudo_class(DOM::Element const& element, Vector
 {
     FlyString element_language;
     for (auto const* e = &element; e; e = e->parent_element()) {
-        auto lang = e->deprecated_attribute(HTML::AttributeNames::lang);
-        if (!lang.is_null()) {
-            element_language = FlyString::from_deprecated_fly_string(lang).release_value_but_fixme_should_propagate_errors();
+        auto lang = e->attribute(HTML::AttributeNames::lang);
+        if (lang.has_value()) {
+            element_language = lang.release_value();
             break;
         }
     }