fix: support empty auth config for anonymous registry

- empty username means caller wants to use no credentials, typically for anonymous registry
- Fixes https://github.com/containerd/cri/issues/1249

Signed-off-by: Nishchay Kumar <mrawesomenix@gmail.com>

pkg/server/image_pull.go

@@ -187,7 +187,8 @@ func ParseAuth(auth *runtime.AuthConfig, host string) (string, string, error) {
 		return user, strings.Trim(passwd, "\x00"), nil
 	}
 	// TODO(random-liu): Support RegistryToken.
-	return "", "", errors.New("invalid auth config")
+	// An empty auth config is valid for anonymous registry
+	return "", "", nil
 }
 
 // createImageReference creates image reference inside containerd image store.