3-0-stable no nexus defined issue

[carlos-serfe]

I've been recently made some tests and found that if the nexus, on the taxjar panel, isn't defined there's an error when checking the taxjar_response.

Worth to mention that I'm using taxjar-ruby (1.7.1) if that's matters.

The issue is that the Spree::Taxjar.new(order).calculate_tax_for_order response is returning 0 instead of nil which is evaluated as true and causes an error later on taxjar_response.breakdown.line_items because the response is an integer and doesn't have the property breakdown.