LibJS: Stop using Optional<Value> in favor of Value's empty state

JS::Value already has the empty state ({} or Value() gives you one.)
Use this instead of wrapping Value in Optional in some places.
I've also added Value::value_or(Value) so you can easily provide a
fallback value when one is not present.

Libraries/LibJS/Runtime/Value.h

@@ -164,6 +164,13 @@ public:
 
     Object* to_object(Heap&) const;
 
+    Value value_or(Value fallback) const
+    {
+        if (is_empty())
+            return fallback;
+        return *this;
+    }
+
 private:
     Type m_type { Type::Empty };