fix nil exception on Client#transactions when user has no transactions

the current gem fails when running the following on a fresh new account (with no transactions):

```
c = Coinbase::Client.new ENV['COINBASE_API_KEY']
c.transactions
> NoMethodError: undefined method `each' for nil:NilClass
```

this is because the reponse to `GET '/transactions'` should include a `"transactions" => []`, but this is field is simply ommited.
this patch adds a check for this and solves the problem (test included)