[Registry] Package release 'metadata' is optional (#3944)
[Registry] Package release 'metadata' is optional (#3944)

Per [API spec](https://github.com/apple/swift-package-manager/blob/main/Documentation/Registry.md#42-fetch-information-about-a-package-release), `metadata` is optional:

> A server MAY include metadata in its package release response.
abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager