title: @NotNull validation on collection with Jackson
body: <p>I have a problem with clearable exception when I post to my controller - json with nulls.</p>
  
  <p>I<code>ve set all fields as @NotNull. And it</code>s work fine (which null values I see nice exception with message: Field error in object '' on field '': rejected value [null]).</p>
  
  <p>Unfortunately collections fields with @NotNull return JsonMappingException.</p>
  
  <p>For example: </p>
  
  <pre><code>  @Value
  public final class User {
  
      @NotNull
      private final Integer age; //work fine
  
  
      @NotNull
      private final List&lt;Books&gt; books; //doesn`t work
  </code></pre>
  
  <p>I<code>ve also tried with @Valid and @NotEmpty, but it didn</code>t help.</p>
  
  <p>My controller method:</p>
  
  <pre><code>@Timed
      @ExceptionMetered
      @PostMapping
      @ResponseStatus(HttpStatus.CREATED)
      @PreAuthorize("@auth.hasAccess(...)")
      void registerUser(@RequestBody @Valid User user) {
          userService.registerUser(user);
      }
  </code></pre>
  
  <p>Do you know what is wrong with it?</p>