adding more error checking for user field in manifest

src/confcom/azext_confcom/security_policy.py

@@ -468,7 +468,7 @@ def populate_policy_content_for_all_images(
 
                     if (
                         not deepdiff.DeepDiff(image.get_user(), config.DEFAULT_USER, ignore_order=True)
-                        and image_info.get("User") != ""
+                        and (image_info.get("User") != "" and image_info.get("User") is not None)
                     ):
                         # valid values are in the form "user", "user:group", "uid", "uid:gid", "user:gid", "uid:group"
                         # where each entry is either a string or an unsigned integer