SWE-12 disable payment buttons in given situations

- if the minimum payment due is $0, we disable the button to allow paying that amount
- if the current account balance is $0, we disable the button to allow paying that amount

SynchronyFinancial/SynchronyFinancial WatchKit Extension/PayBillInterfaceController.swift

@@ -60,6 +60,9 @@ class PayBillInterfaceController: WKInterfaceController {
             let minimumFormatted = String(format: "$%.2f", valid.minPayDue)
             minimumLabel.setText("Minimum Payment:\n\(minimumFormatted)")
             payMinimumButton.setTitle("Pay \(minimumFormatted)")
+
+            payMinimumButton.setEnabled(valid.minPayDue > 0.0)
+            payBalanceButton.setEnabled(valid.curBalance > 0.0)
         }
     }
 }