Describe the bug
When you delete an Additional CSS Class, it leaves an empty className attribute in the HTML comment code.

To Reproduce
Steps to reproduce the behavior:

1. Insert a heading block and type some text
2. In the Advanced panel, set an Additional CSS class
3. Empty the Additional CSS class field
4. Switch to Code Editor.

<!-- wp:heading {"className":""} -->
<h2>Heading</h2>
<!-- /wp:heading -->

Desktop:

• OS: Windows 10, Mac OS
• Browser; Chrome, Safari Firefox, Edge

Additional context
Gutenberg 4.2.0