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/HTML/HTMLLabelElement.h

@@ -18,7 +18,7 @@ public:
 
     virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;
 
-    DeprecatedString for_() const { return deprecated_attribute(HTML::AttributeNames::for_); }
+    Optional<String> for_() const { return attribute(HTML::AttributeNames::for_); }
 
 private:
     HTMLLabelElement(DOM::Document&, DOM::QualifiedName);