core: duplicate note without content

packages/core/models/note.js

@@ -138,10 +138,12 @@ export default class Note {
     return await this._db.notes.add({
       ...qclone(this._note),
       id: undefined,
-      content: {
+      content: content
+        ? {
             type: content.type,
             data: content.data
-      },
+          }
+        : undefined,
       readonly: false,
       favorite: false,
       pinned: false,