Merge branch '19766-dont-crash-when-no-objects-to-cache' into 'master'
ObjectRenderer doesn't crash when no objects to cache with Rails.cache.read_multi ## What does this MR do? Avoid calls to Rails.cache.read_multi without cache keys so it doesn't raise an exception ## What are the relevant issue numbers? Closes #19766 ## Does this MR meet the acceptance criteria? - [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added. I considered is not needed is a fix over a RC - ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~ - ~~[ ] API support added~~ - Tests - [x] Added for this feature/bug - [x] All builds are passing - [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides) - [x] Branch has no merge conflicts with `master` (if you do - rebase it please) - [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits) See merge request !5229
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