fix(datastore): gracefully handle missing Redis cache keys

Related to #461 (closed).

While doing some local tests, I realized that we're logging a warning when a key does not exist on Redis:

WARN[0002] failed to read repository from cache          correlation_id=01G8E2JPFPAKSBB7JKMQ9E4AGQ error="redis: nil" go_version=go1.17.2 version=v3.51.1-gitlab-5-g7cfcb62b.m

This happens because we're not handling the redis.Nil error properly. This MR fixes that.