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