Solved 587-failed-download-history-not-working

Some history items had null as downloadClientId on which FailedDownloadService crashed on that.
This PR is to prevent those crashes.
Note that for all Blackhole/nzbget grabs history items have null as downloadClientId.

But the original complaint was for history downloadFailed events (with null) which had matching grabbed events (with valid id). This hasn't been checked by a dev though.
As far as I can see in the code that can't happen: An existing grabbed.downloadClientId is always copied to the downloadFailed event. Unfortunately this means the 'cause' can't be solved.