pudb barfs if $HOME isn't set

I sometimes need to debug in a reduced-environment ... environment where this variable isn't set.

pudb starts up fine, but subsequently crashes with "Permission denied: /.config".

I'd expect pudb to just skip config persistence in this case, possibly with a warning.

No file or test is named in the issue. Reproduce pudb in an environment without HOME, trace where it attempts to use /.config, and verify that startup succeeds without persistence while providing the proposed warning.

Written by the indexing model from the issue text.