[6.1] Handle paginated registry metadata responses (#8219) (#8262)

Author: plemarquand

Sources/PackageMetadata/PackageMetadata.swift

@@ -352,7 +352,7 @@ public struct PackageSearchClient {
         ) { result in
             do {
                 let metadata = try result.get()
-                let alternateLocations = metadata.alternateLocations ?? []
+                let alternateLocations = metadata.alternateLocations
                 return completion(.success(Set(alternateLocations)))
             } catch {
                 return completion(.failure(error))