Skip filling in cadence start date if disabled in specs

What does this MR do?

Do not fill in cadence start date if it's disabled to fix failure in qa/specs/features/ee/browser_ui/2_plan/iterations/create_group_iteration_spec.rb, see details in #347082 (comment 751947814). It started to fail due to the recent change to disable start date when iteration cadence is in "manual" mode.

E2E Test Failure issue(s)

#347082 (closed)

Check-list

• General code guidelines check-list
  • Code review guidelines
  • Style guides