Update DID resolver `contentMetadata` to return `{}`

[kirahsapong]

contentMetadata should be an empty metadata structure {}

Per the spec:

contentMetadata
If the dereferencing is successful, this MUST be a metadata structure, but the structure MAY be empty.
This structure contains metadata about the contentStream. If the contentStream is a DID document, this MUST be a didDocumentMetadata structure as described in DID Resolution.
If the dereferencing is unsuccessful, this output MUST be an empty metadata structure.