Append to the end of this Json object's array the specified move-insertable json/array/object/number and return a non-const reference to this object. json/array/object/number is left in a valid but unspecified state. If isNull, then the type is changed to array. If an exception is thrown, *this is unaffected. Throw bsl::length_error if size() == maxSize(). The behavior is undefined unless isArray() or isNull().