Summary:
If a column family has 0 levels, then existing `TestCompactFiles(...)` may hit
divide-by-zero. To fix, return early if the cf is empty.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9086
Test Plan: TBD
Reviewed By: ajkr
Differential Revision: D31986799
Pulled By: riversand963
fbshipit-source-id: 48f7dfb2b2b47cfc1315cb71ca80eb230d947f17