benchmark: fix sqlite-is-transaction

The variable deadCodeElimination is declared
but not initialized, making it undefined by default.
When using the &&= operator with an undefined left operand,
the result will always remain undefined regardless of
how many iterations run.

```js
let deadCodeElimination;
deadCodeElimination &&= true
deadCodeElimination // undefined
```

Author: RafaelGSS

benchmark/sqlite/sqlite-is-transaction.js

@@ -16,7 +16,7 @@ function main(conf) {
   }
 
   let i;
-  let deadCodeElimination;
+  let deadCodeElimination = true;
 
   bench.start();
   for (i = 0; i < conf.n; i += 1)