enforec empty list of comments on nil response

backend/app/rest/api/rest_public.go

@@ -38,6 +38,9 @@ func (s *Rest) findCommentsCtrl(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Query().Get("format") {
 		case "tree":
 			tree := rest.MakeTree(maskedComments, sort, s.ReadOnlyAge)
+			if tree.Nodes == nil { // eliminate json nil serialization
+				tree.Nodes = []*rest.Node{}
+			}
 			if s.DataService.IsReadOnly(locator) {
 				tree.Info.ReadOnly = true
 			}

backend/app/rest/tree.go

@@ -111,8 +111,8 @@ func (t *Tree) proc(comments []store.Comment, node *Node, rd *recurData, parentI
 }
 
 // filter returns comments for parentID
-func (t *Tree) filter(comments []store.Comment, fn func(comment store.Comment) bool) (f []store.Comment) {
-
+func (t *Tree) filter(comments []store.Comment, fn func(comment store.Comment) bool) []store.Comment {
+	f := []store.Comment{}
 	for _, c := range comments {
 		if fn(c) {
 			f = append(f, c)

backend/app/store/engine/bolt_accessor.go

@@ -170,6 +170,8 @@ func (b *BoltDB) Find(locator store.Locator, sortFld string) (comments []store.C
 // Last returns up to max last comments for given siteID
 func (b *BoltDB) Last(siteID string, max int) (comments []store.Comment, err error) {
 
+	comments = []store.Comment{}
+
 	if max > lastLimit || max == 0 {
 		max = lastLimit
 	}