Setting single_start (back) to 00:00 should be possible

current situation

• Use case 1:
  • a new period is created and defaults to single_start 00:00:00
  • the new period cannot be applied
• Use case 2:
  • a new period is created
  • a single_start is configured for 01:00:00
  • the period is applied
  • the single_start is changed to 00:00:00
  • the period is applied
  • the change is not reflected in the list

desired behaviour

• Use case 1:
  • a new period is created and defaults to single_start 00:00:00
  • the new period can be applied
• Use case 2:
  • a new period is created
  • a single_start is configured for 01:00:00
  • the period is applied
  • the single_start is changed to 00:00:00
  • the period is applied
  • the change is reflected in the list