Adding null check for error code

[ersaxenas]

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

Issue #, if available:

Description of changes:
Adding null check for errorCode in the method errorCode of AmazonSQSMessagingClientWrapper. Method isJMSSecurityException checks in the Set
SECURITY_EXCEPTION_ERROR_CODES for presence of error code. Set internally calls Objects.requireNonNull which throws Null pointer exception Ref: https://github.com/corretto/corretto-17/blob/4806b202a348e6344c84a89ea23eef026788126a/src/java.base/share/classes/java/util/Objects.java#L209.

This causes original exception to be lost and it never gets reported. Below is an example stacktrace where due to proxy auth issue (Service returned HTTP status code 407 (Service: Sqs, Status Code: 407, Request ID: null)) lib reported null pointer instead of original error:

Exception occurred while acknowledging SQS message:null java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:209)
    at java.base/java.util.ImmutableCollections$SetN.contains(ImmutableCollections.java:937)
    at com.amazon.sqs.javamessaging.AmazonSQSMessagingClientWrapper.isJMSSecurityException(AmazonSQSMessagingClientWrapper.java:385)
    at com.amazon.sqs.javamessaging.AmazonSQSMessagingClientWrapper.handleException(AmazonSQSMessagingClientWrapper.java:364)
    at com.amazon.sqs.javamessaging.AmazonSQSMessagingClientWrapper.deleteMessage(AmazonSQSMessagingClientWrapper.java:109)
    at com.amazon.sqs.javamessaging.acknowledge.UnorderedAcknowledger.acknowledge(UnorderedAcknowledger.java:55)
    at com.amazon.sqs.javamessaging.message.SQSMessage.acknowledge(SQSMessage.java:863)

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[ersaxenas]

@ziyanli-amazon Requesting you to please take a look at this pr.