[data-compaction][leo_storage] Compaction is not executed when some nodes are empty

[windkit]

Description

Put an object to LeoFS and then delete it, one would expect auto compaction will start and reclaim the disk space, but it won't happen

Cause

1. When some storage nodes are empty, Zero division error happens in checking candidates for compaction
   • https://github.com/leo-project/leo_storage/blob/develop/src/leo_storage_watchdog_sub.erl#L272
2. Errors are caught at leo_storage_sub but do not leave any error logs
   • https://github.com/leo-project/leo_watchdog/blob/develop/src/leo_watchdog_sub.erl#L139
   • https://github.com/leo-project/leo_watchdog/blob/develop/src/leo_watchdog_sub.erl#L150

[yosukehara]

@windkit I've fixed this issue. When you have time, please test it.

[windkit]

@yosukehara I can confirm this issue is fixed with the commits.