trackable gc cannot clean empty tablet dir

trackable gc 在处理中间态的空 tablet 目录时，存在bug，无法纳入 gc 流程。
导致在大量连续 split 后，产生的中间 tablet 目录不能被 gc。

The issue reports that trackable GC cannot process empty tablet directories in an intermediate state after repeated splits. No files, tests, or entry points are named, so begin by locating the trackable GC and tablet-directory cleanup code, then reproduce or inspect the split scenario. Done means empty intermediate tablet directories are included in the GC flow.

Written by the indexing model from the issue text.