fix(inbox): check if date is null, adding doc

Author: milos1290

AndroidSDK/src/com/leanplum/LeanplumInbox.java

@@ -316,8 +316,16 @@ public int compare(String firstMessageId, String secondMessageId) {
           if (secondMessage == null) {
             return 1;
           }
+
           Date firstDate = firstMessage.getDeliveryTimestamp();
           Date secondDate = secondMessage.getDeliveryTimestamp();
+          // Message with null date will be moved to the back of the list.
+          if (firstDate == null) {
+            return -1;
+          }
+          if (secondDate == null) {
+            return 1;
+          }
           return firstDate.compareTo(secondDate);
         }
       });

AndroidSDK/src/com/leanplum/NewsfeedMessage.java

@@ -146,11 +146,12 @@ public Date deliveryTimestamp() {
   }
 
   /**
-   * Returns the delivery timestamp of the newsfeed message.
+   * Returns the delivery timestamp of the newsfeed message,
+   * or null if delivery timestamp is not present.
    */
   public Date getDeliveryTimestamp() {
     if (deliveryTimestamp == null) {
-      return new Date();
+      return null;
     }
     return new Date(deliveryTimestamp);
   }