LibWeb: Add ReferrerPolicy::EmptyString

One of the valid enum values for "referrer policy" is the empty string.
Since we're using an enum for this, just add it as an enum value.

Userland/Libraries/LibWeb/ReferrerPolicy/ReferrerPolicy.cpp

@@ -11,6 +11,8 @@ namespace Web::ReferrerPolicy {
 StringView to_string(ReferrerPolicy referrer_policy)
 {
     switch (referrer_policy) {
+    case ReferrerPolicy::EmptyString:
+        return ""sv;
     case ReferrerPolicy::NoReferrer:
         return "no-referrer"sv;
     case ReferrerPolicy::NoReferrerWhenDowngrade:

Userland/Libraries/LibWeb/ReferrerPolicy/ReferrerPolicy.h

@@ -13,6 +13,7 @@ namespace Web::ReferrerPolicy {
 
 // https://w3c.github.io/webappsec-referrer-policy/#enumdef-referrerpolicy
 enum class ReferrerPolicy {
+    EmptyString,
     NoReferrer,
     NoReferrerWhenDowngrade,
     SameOrigin,