Upload empty blob when packing

[shizhMSFT]

The current Pack() method packs a {} config blob for the target OCI manifest.

oras-go/pack.go

Line 82 in 6a09a65

configBytes := []byte("{}")

When using OCI manifest as the medium of the artifact, the content of the config blob is unused. Therefore, it is better to have an empty blob instead of {}. That is,

{
  "digest": "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
  "size": 0
}

[shizhMSFT]

@m5i-work Could you take this item?

[m5i-work]

@shizhMSFT Let me take a look.