title: Default value for optional type 'URL?'
body: <p>I have an extension allowing me to download images to my <code>UITableView</code> from API.</p>
  
  <pre><code>extension UIImageView {
  
      func donwloadImage(from url: String) {
          let urlRequest = URLRequest(url: URL(string: url)!)
          let task = URLSession.shared.dataTask(with: urlRequest) { (data,response,error) in
  
              if error != nil {
                  print(error)
                  return
              }
  
              DispatchQueue.main.async {
                  self.image = UIImage(data: data!)
  
              }
          }
          task.resume()
      }
  
  }
  </code></pre>
  
  <p>The app crashes if <code>imageUrl</code> is <code>nil</code>.</p>
  
  <p>How do I wrap it using <code>??</code> so the <code>UIImageView</code> is just blank if there’s no image?</p>
  
  <p>This is a data model and fetch data function</p>
  
  <pre><code>class Article: NSObject {
  
      var headline: String?
      var date: String?
      var imageUrl: String?
      var desc: String?
      var url: String?
  
  } 
  
  @objc func fetchArticles() {
          let urlRequest = URLRequest(url: URL(string: "https://newsapi.org/v2/everything?pageSize=15&amp;sortBy=publishedAt&amp;apiKey=API_KEY")!)
  
  let task = URLSession.shared.dataTask(with: urlRequest) { (data,response,error) in
  
              if error != nil {
                  print(error)
                  return
              }
  
              self.articles = [Article]()
              do {
                  let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as! [String: AnyObject]
  
                  if let articlesFromJson = json["articles"] as? [[String: AnyObject]] {
                      for articlesFromJson in articlesFromJson {
                          let article = Article()
                          if
                              let title = articlesFromJson["title"] as? String,
                              let publishedAt = articlesFromJson["publishedAt"] as? String,
                              let urlToImage = articlesFromJson["urlToImage"] as? String,
                              let description = articlesFromJson["description"] as? String,
                              let url = articlesFromJson["url"] as? String
  
                          {
  
                              article.headline = title
                              article.date = publishedAt
                              article.imageUrl = urlToImage
                              article.desc = description
                              article.url = url
  
  
                          }
                          self.articles?.append(article) // and so on...
  </code></pre>