events_archiver.rb: Handle an empty meeting_metadata hash from redis

On my server 2.3 alpha, the method metadata_for(meeting_id) gives back {}
(empty Hash). Thus "return if meeting_metadata.nil?" does not occur.

Does @redis.hgetall give {} instead of nil, even though there is a comment in
node_modules/redis/lib/utils.js "hgetall converts its replies to an Object. If
the reply is empty, null is returned"???