Downloads can be empty

[garak]

This is the same problem detected in #80, but for $downloads property.

[JellyBellyDev]

use case, package: happyr/translation-bundle

Cannot assign null to property Packagist\Api\Result\Package::$downloads of type Packagist\Api\Result\Package\Downloads

[robbieaverill]

That example package shows that favers can be null too, will adjust that also (edit: favers is already handled)