Trying to write a cache data $NULL doesn't seem to do anything.

Current Behavior

If there is already a value, it doesn't change.
If no value is set, no file is created.

Expected Behavior

Either write a null value into the file like with any other value,
or delete the file entirely since a non-existing file already returns $NULL.

Alternatively a new Remove-IcingaCacheData function could be added to remove cache data completely.