Provide default values for table view properties which are counted

Summary:
PHP 7.2 has changed the behavior of `count`, you must provide an array or `Countable` as a parameter, otherwise a warning is generated.  These two class members are counted during rendering, and are commonly left as null properties.

https://wiki.php.net/rfc/counting_non_countables

Test Plan: Browsed around my install and stopped seeing `count(): Parameter must be an array or an object that implements Countable at [AphrontTableView.php:153]` everywhere.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D19140

src/view/control/AphrontTableView.php

@@ -4,7 +4,7 @@ final class AphrontTableView extends AphrontView {
 
   protected $data;
   protected $headers;
-  protected $shortHeaders;
+  protected $shortHeaders = array();
   protected $rowClasses = array();
   protected $columnClasses = array();
   protected $cellClasses = array();
@@ -21,7 +21,7 @@ final class AphrontTableView extends AphrontView {
   protected $sortParam;
   protected $sortSelected;
   protected $sortReverse;
-  protected $sortValues;
+  protected $sortValues = array();
   private $deviceReadyTable;
 
   public function __construct(array $data) {