fix get_smartd_schedule_pieces

Providing a valid smartd range in the webUI caused the smartd.conf to be written with empty range information () . The issue is that the value is expected to have a divisor if given a hyphenated range (i.e. 10-15/1 ). However, it's perfectly fine to not have a divisor and just take the values between the hypen. This fixes the scenario when a hyphenated range is given without a divisor. I've also added 2 more unit tests for this scenario.