fix: test if return type is null before using it

Author: pkernevez

modules/openapi-generator/src/main/resources/JavaSpring/apiUtil.mustache

@@ -21,9 +21,11 @@ public class ApiUtil {
     public static void setExampleResponse(NativeWebRequest req, String contentType, String example) {
         try {
             HttpServletResponse res = req.getNativeResponse(HttpServletResponse.class);
-            res.setCharacterEncoding("UTF-8");
-            res.addHeader("Content-Type", contentType);
-            res.getWriter().print(example);
+            if (res != null) {
+                res.setCharacterEncoding("UTF-8");
+                res.addHeader("Content-Type", contentType);
+                res.getWriter().print(example);
+            }
         } catch (IOException e) {
             throw new RuntimeException(e);
         }