Fix add bookmark with no tags

lib/models/data/post_bookmark.dart

@@ -17,13 +17,21 @@ class PostBookmark {
     required this.tagNames,
   });
 
-  Map<String, dynamic> toJson() => {
+  Map<String, dynamic> toJson() {
+    Map<String, dynamic> ret = {
       "url": url,
       "title": title,
       "description": description,
       "is_archived": isArchived,
       "unread": unread,
       "shared": shared,
+    };
+    if (tagNames != "") {
+      ret = {
+        ...ret,
         "tag_names": tagNames,
       };
     }
+    return ret;
+  }
+}