[ZD#708471] fix TypeError in handleGetOrders

#bug

[App "Bigcommerce"] TypeError: Cannot read property '0' of undefined
    at AppSubclass.source.handleGetOrders (installed.js:149:29)

Bigcommerce returns 204 :crying_cat_face: when there are no orders so data will be undefined.

/cc @maximeprades 

#### References:

- [[ZD#708471] BigCommerce Integration](https://support.zendesk.com/agent/#/tickets/708471)