fix aggregation logic

internal/report/snapshots.go

@@ -464,6 +464,8 @@ func recordsFromTableNames(ctx context.Context, database db.Database, snapshotTy
 				TableName:    table,
 				SnapshotTime: ts,
 				SnapshotType: snapshotType,
+				// Unknown row count when snapshot_registry isn't available.
+				SnapshotCount: -1,
 			})
 		}
 	}