oeqa/selftest/archiver: Allow tests to ignore empty directories

If we tweak sstate to not remove empty directories under conditions
where a race could occur, we see failures from:

"oe-selftest -r archiver.Archiver.test_archiver_filters_by_type archiver.Archiver.test_archiver_filters_by_type_and_name"

since an empty directory is left behind. Update the tests to ignore
empty directories.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>