Linux: Empty environment variables should use defaults

[domdfcoding]

This was originally reported in the appdirs tracker: ActiveState/appdirs#130

The gist is that, per the XDG spec, platformdirs should treat environment variables that are set but empty as if they were unset.

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I came across this as part of pypa/pip#10202

pip patches appdirs to work around this: https://github.com/pypa/pip/blob/867bbb00ccb0506095e19a0e111ce06ce71a2495/tools/vendoring/patches/appdirs.patch#L47-L54

There is some prior discussion in pypa/pip#7501 (comment)

[gaborbernat]

Can you open a PR to address it? Thanks!

[papr]

I will give this a try.