fix: check if zfs-cache exists and isnt empty (#409)

- check the cache file exists, and isnt empty
- resolves idempotence for zpool-import-cache service

Reviewed-on: #409

modules/zfs/lib/facter/zfs_zpool_cache_present.rb

@@ -3,7 +3,8 @@
 Facter.add('zfs_zpool_cache_present') do
   confine kernel: 'Linux'
   setcode do
-    File.exist?('/etc/zfs/zpool.cache')
+    cache_file = '/etc/zfs/zpool.cache'
+    File.exist?(cache_file) && File.size(cache_file).positive?
   end
 end