Markdown stability patch

[demshy]

Summary

Trying to address some of the reproducible bugs in current markdown editor. The ultimate plan is still to upgrade the editor to Plate and have high hopes that that will fix most of the problems, but in the meantime we can try to patch some of the most annoying ones.

First fix is for #7220 - where converting markdown without text (but newlines, breaks or spaces) would result in a slate root without paragraph node.

Second fix is for #7123 - a case where backspace in urls contained in lists breaks the editor

Test plan
#7220
Switch to markdown mode, add a space or a line break, switch back to richtext mode and hit shift+enter. The editor should no longer crash.

#7123
Create a bullet list with two items. In the second change some of the text to link. Then use backspace at the start of that link. The editor should not crash. Keep using backspace until the bullet is removed and the editor should also not crash then.

Checklist

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• I have read the [contribution guidelines](https://github.com/decaporg/decap-cms/blob/main/CONTRIBUTING.md).

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