LibJS: Make the default constructed reference invalid

Since we have the to_reference method on every expression class we must
somehow communicate it did not actually return a reference.
This (ab)uses the fact that property name is only invalid with the
default constructor and already has is_valid().

Userland/Libraries/LibJS/Runtime/Reference.h

@@ -109,6 +109,8 @@ public:
 
     String to_string() const;
 
+    bool is_valid_reference() const { return m_name.is_valid(); }
+
 private:
     void throw_reference_error(GlobalObject&) const;