ArchFxStreamSlug cannot handle an empty parent

A few tests that set the parent as `''` or `None` are failing. e.g.

https://github.com/iotile/python_archfx_cloud/blob/main/tests/test_slugs.py#L147

Need to fix that and reenable those tests. Passing `''` or `None` should be equivalent to passing a 0 and should represent `pl--0000-0000`

Note that we may have fixed that already on archfx_cloud: https://github.com/iotile/archfx_cloud/blob/master/server/apps/utils/gid/slugs.py

so need to diff with that file and update as needed. Once we deploy, the archfx_cloud should be replaced with this package