Skip empty CIDs in repair cycle

[raymondjacobson]

See lots of logs like this, will investigate later, but for now a good check to have b/c the next calls will necessarily fail

{
  "cid": "",
  "cleanup": true,
  "error": "blob (code=Unknown): operation error S3: HeadObject, serialization failed: serialization failed: input member Key must not be empty",
  "errorVerbose": "blob (code=Unknown):\n    gocloud.dev/blob.(*Bucket).Attributes\n        /go/pkg/mod/gocloud.dev@v0.40.0/blob/blob.go:919\n  - operation error S3: HeadObject, serialization failed: serialization failed: input member Key must not be empty",
  "level": "error",
  "msg": "exist check failed",
  "node": "https://audius-discovery-3.altego.net",
  "service": "mediorum",
  "task": "repair"
}