[Bug]: Empty fleets are not deleted

[peterschmidt85]

Steps to reproduce

1. Create a fleet YAML config with idle duration set to 1m
2. Create the fleet
3. Wait 1m

Actual behaviour

1. Instances are deleted due to idle duration
2. The empty fleet remains shown in the list of fleets

Expected behaviour

1. The empty fleet should be automatically deleted

dstack version

Any

Server logs

No response

Additional information

No response