Skip "null" lines when constructing raw documents for DocumentEngine rendering

Summary: See https://discourse.phabricator-community.org/t/more-exceptions-when-viewing-diffs/3789. This is a similar issue with the same datastructure.

Test Plan: Works correctly under PHP 7.3, but this may not be the end of things.

Differential Revision: https://secure.phabricator.com/D21171