Fix falsly setting autoTransaction

[dertieran]

When passing a falsy value to transacting a new transaction is created but autoTransaction is set to false so the transaction is never committed or rolled back.

This is a problem because this means batchInsert works different then insert here.
So passing null into transacting works for insert

knex.insert(rows)
    .into('TableName')
    .transacting(null)

but it doesn't for batchInsert

knex.batchInsert('TableName', rows, chunkSize)
    .transacting(null)

[wubzz]

Thanks!