ObjectRenderer read_multi raises and exception when there is no cacheable entries
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5096#note_13028382
On !5096 (merged) we added the option to get renderered content from the cache in one call but we introduced a bug when the provided object has no cache key to be stored so they won't be cache or read from the cache
Rails.cache.read_multi([])
NoMethodError: undefined method `map!' for nil:NilClass