There are a few different ways you could address this. Here's one:

func dataFetcherDevice<T: Decodable>(url: String, completion: @escaping ([T]) -> Void) {
    //this takes in result, which is of type [T]?
    networkDataFetcher.dataFetcher(url: url) { result in
        //and gives back to the completion handler either a filled in [T] or an empty array
        completion(result ?? [])
    }
}

So instead of just passing along the completion handler to the inner function call, you nest completion handlers.

Of course, it would probably just be easier to have NetworkDataService.dataFetcher take a completion handler of type ([T]) -> Void in the first place and return [] in the event of failure rather than nil.