Make `zpool import -d|-c` behave consistently

When importing pools with zpool import -aN there is inconsistent
behavior between '-d /dev/disk/by-id' (or another path) and
'-c /etc/zfs/zpool.cache'.

The difference in behavior is caused by zpool_find_import_cached()
returning an empty nvlist_t when there are no pools to import but
zpool_find_import_impl() returns NULL for the same situation. The
behavior of zpool_find_import_cached() is arguably more correct
because it allows returning NULL to be used for an error case and
not an empty set.

This change resolves the issue by updating get_configs() such that
it returns an empty set instead of NULL when no config is found.
The updated behavior will now always return 0 for this case.

$ zpool import -aN; echo $?
no pools available to import
0

$ zpool import -aN -d /var/tmp/; echo $?
no pools available to import
0

$ zpool import -aN -c /etc/zfs/zpool.cache; echo $?
no pools available to import
0

Signed-off-by: Brian Behlendorf <[email protected]>
Closes #2080

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