Werk #12687: activation_cleanup: fix growing tmp directory due to inactive cleanup job

The "Activate Changes" functionality creates a number of temporary directories under tmp, which are periodically removed by a cleanup job "Activation Cleanup". This job was not able to repair itself in the event that it had a broken or empty status file: ~/var/check_mk/background_jobs/activation_cleanup/jobstatus.mk. This has been fixed, so that the job can continue to run.

To the list of all Werks