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);