When an admin deletes a package set of which a package is linked to a task then it fails

[Atala]

The task is linked to the package The package set deletion cascades to the packages Deletion of the package fails because it is is linked to a task

https://coopcycle.sentry.io/issues/5288895928/?project=5294179&query=is%3Aunresolved&referrer=issue-stream&statsPeriod=24h&stream_index=0

-> so when we delete package set it should SET_NULL on package but not delete PACKAGE

TO TEST with package set named 1 on eraman instance

[Atala]

linked to #4199

[Paul-Eraman-CoopCycle]

🥳