cart total null fix for braintree payment

app/src/main/java/ch/pizzaleu/android/model/cart/CartTotalModel.java

@@ -17,6 +17,9 @@ public class CartTotalModel implements Serializable {
     }
 
     public String getTitle() {
+        if(title == null){
+            title = "";
+        }
         return title;
     }
 
@@ -25,6 +28,9 @@ public class CartTotalModel implements Serializable {
     }
 
     public String getText() {
+        if(text == null){
+            text = "";
+        }
         return text;
     }