Fix NullPointerException when publishing an SNS batch without a topic ARN

[anjeongkyun]

SnsInterceptor reads the topic ARN to derive the topic name for DSM. The single-publish path already handles a missing ARN (a publish to a phone number has neither topicArn nor targetArn), but the batch path dereferences it straight away:

String snsTopicArn = request.topicArn();
String snsTopicName = snsTopicArn.substring(snsTopicArn.lastIndexOf(':') + 1);

PublishBatchRequest builds fine without a topic ARN, and the interceptor runs before marshalling, so a request that AWS would have rejected with a validation error instead fails with an NPE thrown from the agent. Neither SDK wraps exceptions from modifyRequest/beforeMarshalling, so it surfaces to the caller as-is.

Without the change, the new test fails like this:

Expected exception of type 'software.amazon.awssdk.core.exception.SdkException',
but got 'java.lang.NullPointerException'
Caused by: java.lang.NullPointerException: Cannot invoke "String.lastIndexOf(int)"
  because "snsTopicArn" is null
	at datadog.trace.instrumentation.aws.v2.sns.SnsInterceptor.modifyRequest

Both the v1 and v2 interceptors have it, so both are fixed the same way the single-publish path already does: skip the injection and leave the request alone.

I checked the other ARN parsing in the aws-java modules while I was here. AwsSdkClientDecorator guards it in v1 (if (null != topicArn)) and uses Optional.map in v2, so the two batch branches were the only unguarded ones.

Tests mirror the existing SNS message to phone number doesn't leak exception case. aws-java-sns-1.0 and aws-java-sns-2.0 test tasks pass, spotlessApply is clean.

Could a maintainer add inst: aws and type: bug? I can't set labels on this repo.