Error in commerce checkout complete with Stats module enabled and Campaign module disabled

I found a bug that causes a PHP error (or "white screen" when errors are not visible).

Conditions to reproduce:

• Install Drupal Commerce
• Install MailJet and the MailJet_stats submodule
• Don't install the mailjet_campaign module

The error:
When you proceed through checkout and would normally get to the Checkout Complete screen, that's when the error occurs: Fatal error: Call to undefined function mailjet_campaign_load(). That's because the function mailjet_stats_commerce_checkout_complete() is calling mailjet_campaign_load(), but the mailjet_campaign module isn't installed, and therefore the function isn't available.

The fix:
Add a check such as function_exists() or module_exists().