Apply suggestions from code review

Co-authored-by: Stuart McCulloch <stuart.mcculloch@datadoghq.com>

Author: amarziali

dd-java-agent/instrumentation/aws-java/aws-java-sqs-1.0/src/main/java/datadog/trace/instrumentation/aws/v1/sqs/TracingIterator.java

@@ -69,7 +69,7 @@ protected void startNewMessageSpan(Message message) {
     try {
       if (InstrumenterConfig.get().isLegacyContextManagerEnabled()) {
         closePrevious(true);
-      } else {
+      } else if (message == null) { // previous message span was the last
         final AgentSpan previousSpan = spanFromContext(getRootContext().swap());
         if (previousSpan != null) {
           previousSpan.finishWithEndToEnd();

dd-java-agent/instrumentation/aws-java/aws-java-sqs-2.0/src/main/java/datadog/trace/instrumentation/aws/v2/sqs/TracingIterator.java

@@ -71,7 +71,7 @@ protected void startNewMessageSpan(Message message) {
     try {
       if (InstrumenterConfig.get().isLegacyContextManagerEnabled()) {
         closePrevious(true);
-      } else {
+      } else if (message == null) { // previous message span was the last
         final AgentSpan previousSpan = spanFromContext(getRootContext().swap());
         if (previousSpan != null) {
           previousSpan.finishWithEndToEnd();