Backspace doesn't delete empty line

[m2-farzan]

Bug description:

In a code cell, pressing backspace removes characters correctly until it reaches the beginning of the line. Then, it fails to delete the empty line and go up.

I stepped through the recent commits and tested them. Apparently d406b8c is causing the problem.

[jasongrout]

Thanks for running master and finding the commit. Assigning to the 6.0.4 milestone so we don't ship broken code.

[kevin-bates]

@benthayer - would you be able to look into this as it appears related to #4880?

[benthayer]

Yes, I'll check it out this weekend. I think this code is pretty similar to the code I used to fix this in jupyterlab/jupyterlab#7401, so I'll check to see if that's an issue as well