BGP notification system test issues fix

Change-Id: I53b1aca6cf361fa212dd5ab83380bef40bbc5cd7

@@ -194,7 +194,9 @@ class BgpNotificationMsgVer4 implements BgpNotificationMsg {
             }
             cb.writeByte(message.errorCode);
             cb.writeByte(message.errorSubCode);
-            cb.writeBytes(message.data);
+            if (message.data != null) {
+                cb.writeBytes(message.data);
+            }
 
             //Update message length field in notification message
             int length = cb.writerIndex() - msgStartIndex;

@@ -166,6 +166,11 @@ public class BgpPathAttributes {
     public static void checkMandatoryAttr(boolean isOrigin, boolean isAsPath,
             boolean isNextHop, boolean isMpReach, boolean isMpUnReach)
             throws BgpParseException {
+        // Mandatory attributes validation not required for MP_UNREACH
+        if (isMpUnReach) {
+            return;
+        }
+
         if (!isOrigin) {
             log.debug("Mandatory Attributes not Present");
             Validation.validateType(BgpErrorType.UPDATE_MESSAGE_ERROR,