message: nydusify: fix conversion for the image included empty platform
  If the image includes an empty platform manifest, nydusify will panic on
  conversion, check this situation to avoid.
diff: @@ -261,7 +261,7 @@ func (remote *Remote) Pull() error {
  platform := platforms.Default()
  var sourceManifestDesc *ocispec.Descriptor
  for _, desc := range descs {
  - if platform.Match(*desc.Platform) {
  + if desc.Platform == nil || platform.Match(*desc.Platform) {
  sourceManifestDesc = &desc
  break
  }
  