zfs-import-{cache,scan}: change condition to FileNotEmpty

When all pools are exported ZFS will generate an empty cache file.
This will cause the import service to fail, which is sub-optimal,
since this means that dracut fails, and it necessary to run
zpool import -a to boot, delete the file, and regenerate+reinstall
the initrd.

This resolves the issue by treating an zero-length cache files the
same as a missing cache file. This aligns the behavior with that
of the zpool command itself.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11568