url field is always empty even after inserting/upserting

[StewartJingga]

Describe the bug
When upserting an asset via [PUT | PATCH] /v1beta1/assets and fetching it back via [GET] /v1beta1/assets/{urn}, the url field is always empty even when we already define it when upserting.

[PATCH] /v1beta1/assets

{
  "urn": "sample-urn",
  "url": "http://sample-url.com"
}

[GET] /v1beta1/assets/sample-urn

{
  "urn": "sample-urn"
}

To Reproduce
Steps to reproduce the behavior:

1. Upsert an asset via [PUT | PATCH] /v1beta1/assets with filled url
2. Fetch the same asset via [GET] /v1beta1/assets/{urn}
3. See url is still empty

Expected behavior
url should be returned if it has been upserted.

[sudo-suhas]

Closed via #209