PHP notice "Undefined index: payment_method" if an order doesn't have a payment method

Orders don't necessarily have a payment method when checkout is completed, e.g. an order with zero total may be programmatically completed through checkout with commerce_checkout_complete().

commerce_stripe_pi_commerce_checkout_complete() doesn't check for a payment method array key in $order->data before attempting to access it, resulting in

Notice: Undefined index: payment_method in commerce_stripe_pi_commerce_checkout_complete() (line 922 of /var/www/html/sites/all/modules/contrib/commerce_stripe_pi/commerce_stripe_pi.module). 

Simple fix:

-  if ($order->data['payment_method'] !== 'commerce_stripe_pi|commerce_payment_commerce_stripe_pi') {
+  if (!isset($order->data['payment_method']) || $order->data['payment_method'] !== 'commerce_stripe_pi|commerce_payment_commerce_stripe_pi') {
     return;
   }