Fixed creation of JsonVariant::_invalid

include/ArduinoJson/JsonVariant.hpp

@@ -166,6 +166,9 @@ class JsonVariant : public Internals::JsonPrintable<JsonVariant> {
   JsonVariant &operator[](const char *key);
 
  private:
+  // Special constructor used only to create _invalid.
+  JsonVariant(Internals::JsonVariantType type) : _type(type) {}
+
   // Helper for interger cast operators
   template <typename T>
   T cast_long_to() const {