title: REST API: resource needs to be filtered by ID, but no ID specified
body: <p>Let's say I have a resource <code>/books</code>, with a parameter <code>?author_id=</code>. What should be returned if no <code>author_id</code> parameter is specified, e.g., <code>/books</code> as opposed to <code>/books?author_id=42</code>? Based partially on <a href="https://stackoverflow.com/a/13367198/3642398">this answer</a>, I have decided that if you hit <code>/books?author_id=42</code> you will get an empty collection if author <code>42</code> has no books, instead of e.g., returning a 404. </p>
  
  <p>However, in the case that <code>client_id</code> is null, I am not sure if I should treat this case the same way (i.e., the author with the ID of null has no books) or if it should be something like a 400, treating null as an invalid value for <code>client_id</code>. </p>
  
  <p>Note also that <code>books</code> is a contrived example, and that in my case, I cannot simply return all "books" if the user hits <code>/books</code>. Assume it is a case that only the author has access to their books. </p>