LibWeb: Consistently use the EmptyString state of ReferrerPolicy

We previously used an empty optional to denote that a ReferrerPolicy is
in the default empty string state. However, later additions added an
explicit EmptyString state. This patch moves all users to the explicit
state, and stops using `Optional<ReferrerPolicy>` everywhere except for
when an option not being passed from JavaScript has meaning.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -107,7 +107,7 @@ private:
     CORSSettingAttribute m_crossorigin { CORSSettingAttribute::NoCORS };
 
     // https://html.spec.whatwg.org/multipage/scripting.html#attr-script-referrerpolicy
-    Optional<ReferrerPolicy::ReferrerPolicy> m_referrer_policy;
+    ReferrerPolicy::ReferrerPolicy m_referrer_policy { ReferrerPolicy::ReferrerPolicy::EmptyString };
 
     bool m_failed_to_load { false };