Allow iteration to be created without cadence

What does this MR do and why?

Resolves #354085 (closed).

After turning off iteration_cadences FF, users should be able to create new iterations. Because each iteration needs to belong to an iterations_cadence internally, we will assign a new iteration to its group's first cadence.

How to set up and validate locally

1. Create a new group

2. Turn on :iteration_cadences

3. Create two new cadences

4. Turn off :iteration_cadences

5. Confirm that a new iteration can be created.

6. Turn on :iteration_cadences

7. Confirm that the new iteration's been placed in the first cadence.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.