never return null for collections in describeRepo (#7)

in a repo with no records, the collections field here ends up being the
nil slice instead of an empty slice, so you get null in the response.

since collections is a required field in the rpc def we shouldn't do this,
it breaks pdsls at least.

server/handle_repo_describe_repo.go

···
 		return helpers.ServerError(e, nil)
 	}
 
-	var collections []string
+	var collections []string = make([]string, 0)
 	for _, r := range records {
 		collections = append(collections, r.Nsid)
 	}