Updating a note removes the title

When I use `hackmd-cli notes update --noteId="XYZ" --content="New content"` to set the contents of a note, the title of the note is removed. Trying to include the title of the note in the content, e.g.

```
hackmd-cli notes update --noteId="XYZ" --content="# My title\nNew content"
```

does not solve the issue: instead, the `# My title` part is included in the content of the note and the title is still empty.

Start by tracing the `notes update` command in the TypeScript CLI and follow how its content is sent and the note title is handled. Reproduce the command shown in the issue, then verify that updating content preserves the existing title while replacing the note content.

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