For nullable types (i.e. strings)
employee.FirstName = sqlreader[indexFirstName] as string;
For primitive values (i.e. integer), if you cast to a nullable int, you can use GetValueOrDefault()
employee.Age = (sqlreader[indexAge] as int?).GetValueOrDefault();
or the null-coalescing operator (??).
employee.Age = (sqlreader[indexAge] as int?) ?? 0;