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/HTMLInputElement.h

@@ -111,7 +111,7 @@ public:
     virtual bool is_focusable() const override { return m_type != TypeAttributeState::Hidden; }
 
     // ^HTMLElement
-    virtual void attribute_changed(FlyString const&, DeprecatedString const&) override;
+    virtual void attribute_changed(FlyString const&, Optional<DeprecatedString> const&) override;
 
     // ^FormAssociatedElement
     // https://html.spec.whatwg.org/multipage/forms.html#category-listed