5159: Fix the New Indexer Spilling r=irevoire a=Kerollmops
Fix two bugs in the merging of the spilled caches. Thanks to `@ManyTheFish` and `@irevoire` 👏
Co-authored-by: Kerollmops <clement@meilisearch.com>
Co-authored-by: ManyTheFish <many@meilisearch.com>