Servlet: avoid NPE when headerNames is null (#11394)

Servlet: avoid NPE when headerNames is null

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: amarziali

dd-java-agent/instrumentation/servlet/javax-servlet/javax-servlet-3.0/src/main/java/datadog/trace/instrumentation/servlet3/HttpServletExtractAdapter.java

@@ -30,7 +30,8 @@ public static final class Request extends HttpServletExtractAdapter<HttpServletR
 
     @Override
     Enumeration<String> getHeaderNames(HttpServletRequest request) {
-      return request.getHeaderNames();
+      final Enumeration<String> ret = request.getHeaderNames();
+      return ret != null ? ret : emptyEnumeration();
     }
 
     @Override

dd-java-agent/testing/src/main/java/datadog/trace/agent/test/server/http/HttpServletRequestExtractAdapter.java

@@ -20,6 +20,9 @@ public class HttpServletRequestExtractAdapter
   public void forEachKey(
       final HttpServletRequest carrier, final AgentPropagation.KeyClassifier classifier) {
     Enumeration<String> headerNames = carrier.getHeaderNames();
+    if (headerNames == null) {
+      return;
+    }
     while (headerNames.hasMoreElements()) {
       final String header = headerNames.nextElement();
       if (!classifier.accept(header, carrier.getHeader(header))) {