Provide default values for table view properties which are `count`ed
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