nitpicks from yesterday's changes (#9)

* µopt: only do one allocation here

* just check if preferences is a key of the prefs obj

i think json.Unmarshal can give u a non-nil guy here and checking for
nil or empty is kinda cringe so let's just do this

server/handle_actor_get_preferences.go

···
 
 	var prefs map[string]any
 	err := json.Unmarshal(repo.Preferences, &prefs)
-	if err != nil || prefs == nil {
+	if err != nil || prefs["preferences"] == nil {
 		prefs = map[string]any{
 			"preferences": map[string]any{},
 		}

server/handle_repo_describe_repo.go

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