Backspace at the header beginning should keep the format

[luin]

Currently, press backspace at the beginning of a header clears the header format even if the previous line is empty.

To reproduce

1. In an empty editor, press the enter key to create a new line.
2. Enter some text and set the line as a header.
3. Place the cursor at the beginning of the line.
4. Press backspace.

Actual

The header format is lost.

Expected

The header format should be kept. In contrast, if the previous line is not empty (either it has content or it is set a block format like { header: 2 }).

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Test plan

Follow the reproduce steps, make sure the result is what mentioned in the expected section above.