UpdateCalendarResourcesRoomsBackgroundJob fails if no backend available

[juliusknorr]

@georgehrke This should not fail if no backend is available, i guess.

Return value of OC\Calendar\Resource\Manager::getBackends() must be of the type array, null returned

{"reqId":"uo34yCA4vy4szC8muPYg","level":3,"time":"2018-07-11T16:28:38+00:00","remoteAddr":"","user":"--","app":"cron","method":"","url":"--","message":{"Exception":"TypeError","Message":"Return value of OC\\Calendar\\Resource\\Manager::getBackends() must be of the type array, null returned","Code":0,"Trace":[{"file":"\/var\/www\/html\/apps\/dav\/lib\/BackgroundJob\/UpdateCalendarResourcesRoomsBackgroundJob.php","line":96,"function":"getBackends","class":"OC\\Calendar\\Resource\\Manager","type":"->","args":[]},{"file":"\/var\/www\/html\/apps\/dav\/lib\/BackgroundJob\/UpdateCalendarResourcesRoomsBackgroundJob.php","line":88,"function":"runResources","class":"OCA\\DAV\\BackgroundJob\\UpdateCalendarResourcesRoomsBackgroundJob","type":"->","args":[]},{"file":"\/var\/www\/html\/lib\/private\/BackgroundJob\/Job.php","line":61,"function":"run","class":"OCA\\DAV\\BackgroundJob\\UpdateCalendarResourcesRoomsBackgroundJob","type":"->","args":[null]},{"file":"\/var\/www\/html\/lib\/private\/BackgroundJob\/TimedJob.php","line":55,"function":"execute","class":"OC\\BackgroundJob\\Job","type":"->","args":[{"__class__":"OC\\BackgroundJob\\JobList"},{"__class__":"OC\\Log"}]},{"file":"\/var\/www\/html\/cron.php","line":123,"function":"execute","class":"OC\\BackgroundJob\\TimedJob","type":"->","args":[{"__class__":"OC\\BackgroundJob\\JobList"},{"__class__":"OC\\Log"}]}],"File":"\/var\/www\/html\/lib\/private\/Calendar\/Resource\/Manager.php","Line":60,"CustomMessage":"--"},"userAgent":"--","version":"14.0.0.10"}

[nickvergessen]

https://github.com/nextcloud/server/blob/master/lib/private/Calendar/Resource/Manager.php#L31 is missing = []