Do not use null parent context on tomcat advice (#11968)

Do not use null parent context on tomcat advice

enforce nonnull checks

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

Author: amarziali

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/instrumentation/decorator/HttpServerDecorator.java

@@ -166,7 +166,7 @@ public Context extract(REQUEST_CARRIER carrier) {
    * @param parentContext The parent context of the span to create.
    * @return A new context bundling the span, child of the given parent context.
    */
-  public Context startSpan(REQUEST_CARRIER carrier, Context parentContext) {
+  public Context startSpan(REQUEST_CARRIER carrier, @Nonnull Context parentContext) {
     String instrumentationName = component().toString();
     AgentSpanContext extracted = getExtractedSpanContext(parentContext);
     // Call IG callbacks

dd-java-agent/instrumentation/tomcat/tomcat-5.5/src/main/java/datadog/trace/instrumentation/tomcat/TomcatServerInstrumentation.java

@@ -151,7 +151,7 @@ public static void onService(
       }
       final Object parentContextObj = req.getAttribute(DD_PARENT_CONTEXT_ATTRIBUTE);
       final Context parentContext =
-          (parentContextObj instanceof Context) ? (Context) parentContextObj : null;
+          (parentContextObj instanceof Context) ? (Context) parentContextObj : rootContext();
 
       final Context context = DECORATE.startSpan(req, parentContext);
       serverScope = context.attach();