Cleaned notebooks often fail to debug properly

At least in VS Code, debugging fails semi-silently, when cells' json attributes are set to "execution_count": null, as nbdev_clean does. It appears that this can be worked around by simply setting them to 0 instead of null. Not sure if this will be fixed anytime soon in microsoft's code.

See: https://github.com/microsoft/debugpy/issues/1669#issuecomment-2430200548

Not sure if there are other reasons to use null however.