FileHooks: Prepare to postWrite event with no file owner
FileHooks: Prepare to postWrite event with no file owner

Nextcloud 16 and later seem to now emit postWrite events with file node
without an owner. This didn't happen earlier, so proabably some core patch
has caused this. This happens at least while setting up Travis tests, but
maybe in some other scenarios, too. Our hook code is now prepared for such
events, preventing the unhandled exception.

owncloud/music,paulijar/music,paulijar/music,paulijar/music,owncloud/music,paulijar/music,owncloud/music,paulijar/music,owncloud/music,owncloud/music