Displaying discussions with a note without author will result to error

Summary

Reported on sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/2764126/events/76893031d4384d2e98a5edb392f019a3/.

When requesting for Projects::MergeRequestsController#discussions action and one of the notes has no author (for some reason), it'll result to a NoMethodError. This is because in Note#post_processed_cache_key we get the cache key of the author.

Possible fixes

If note has no author, don't get its cache key so it won't result to an error.