Backspace (joinBackward command) breaks list weird when called on first item of nested list

[andrey-skl]

To reproduce,

1. Open ProseMirror example https://prosemirror.net/examples/markdown/
2. Paste this content in Markdown mode

* one
  * two

3. Switch to WYSIWYG mode
4. Put cursor at the start of first item
5. Hit "Backspace" or click "lift out of enclosing block" toolbar button
   

Expected: the text of firs item is "lifted out" of the list:

Actual: List gets into broken state:

[marijnh]

The list isn't in a broken state. It's a list item that contains another list. Markdown allows this (* * two).