title: display NetworkImageView's default image without network request
body: <p>I have a ListView displaying things, many of which have an illustrative image and some of which don't. The ViewHolder uses NetworkImageView to display the illustrations. </p>
  
  <p>The problem is that even if a default image is set, it won't be displayed until one calls setImageUrl(), <a href="https://android.googlesource.com/platform/frameworks/volley/+/jb-mr2-release/src/com/android/volley/toolbox/NetworkImageView.java">which in turn will set the source bitmap of the view to null</a> if the url is empty. If the url isn't empty, it makes a request. This means that one is forced to make a network request even if there isn't a valid network url associated with that particular view, otherwise instead of displaying the default image it displays an empty view. </p>
  
  <p>(issue filed: <a href="https://code.google.com/p/android/issues/detail?id=59538">https://code.google.com/p/android/issues/detail?id=59538</a>) </p>
  
  <p>Is there a way around making bogus network requests for items without a valid url? </p>