Update the batch validation config parsing so
that the tx batch validation is false by default.
This means that to enable transaction batch validation,
the env var `VALIDATE_TX_BATCH` must be explicitly set
to `true`. If it is not set, then it will default to false.
The logic is shown below for the implementation.

```js
> '' ? '' === 'true' : false
false
> 'false' ? 'false' === 'true' : false
false
> 'true' ? 'true' === 'true' : false
true
```