LibJS: Make Value::m_type default to Type::Empty

This is not effectful since all constructors overwrite the type anyway,
but it seems reasonable that the default value of m_type would match
what Value() would give you.

Libraries/LibJS/Runtime/Value.h

@@ -152,7 +152,7 @@ public:
     Object* to_object(Heap&) const;
 
 private:
-    Type m_type { Type::Undefined };
+    Type m_type { Type::Empty };
 
     union {
         bool as_bool;