Adding null check for error code

Fixing null pointer exception which is occurs when error code is null. This cause original exception to be lost.

Author: ersaxenas

src/main/java/com/amazon/sqs/javamessaging/AmazonSQSMessagingClientWrapper.java

@@ -377,7 +377,7 @@ private JMSException handleException(SdkException e, String operationName) {
     }
 
     private static String errorCode(AwsServiceException e) {
-        return e.awsErrorDetails() != null ? e.awsErrorDetails().errorCode() : "";
+        return e.awsErrorDetails() != null && e.awsErrorDetails().errorCode() != null ? e.awsErrorDetails().errorCode() : "";
     }