Error 400 when trying to save a post with an empty Classic block as only content

[afercia]

Gutenberg allows to save posts with empty blocks, for example:

• create a new post
• leave the title empty
• add a paragraph and leave it empty
• save as draft
• saving is successful
• add an Image block but don't add an image
• add a List block and leave it empty
• save as draft
• saving is successful

Instead:

• create a new post
• leave the title empty
• add a Classic block and leave it empty
• click on Save draft
• you will get an "Updating failed" warning (see screenshot below)

Actually, for a brief moment you will see the Save draft button changed in "Saved" together with the "Updating failed" warning. A bit confusing:

After that, "Saved" turns back into "Save Draft":

The API response is:
{"code":"empty_content","message":"Content, title, and excerpt are empty.","data":{"status":400}}

Instead, saving a post with only an empty paragraph returns the whole post data, with an empty paragraphs as content: "<p><\/p>\n"

[danielbachhuber]

Duplicate of #6556