Use array() instead of null as default value im constructor

Summary:
Variables of type array() should get `array()` as default
value instead of null. That has the benefit that specific array functions
can be called without checking if the variable if really of type array before.

Reviewers: claus.due, #forgetit-dev

Reviewed By: claus.due, #forgetit-dev

Differential Revision: https://phabricator.dkd.de/D433