Make a no-op compaction really a no-op compaction

yama/src/pile/local_sqlitebloblogs.rs

@@ -636,6 +636,11 @@ impl SqliteBloblogPile {
             pub chunk_id: ChunkId,
         }
 
+        if plan.bloblogs_to_replace.is_empty() {
+            info!("No compaction to be done.");
+            return Ok(());
+        }
+
         let mut to_preserve = BTreeSet::new();
         let mut replacements = BTreeMap::new();