R1:40d2346f2965

Add a missing "null" check when rebuilding old/new diff content

Summary:
See <https://discourse.phabricator-community.org/t/exceptions-when-viewing-diffs/3787>. This list may include `null` values.

Until PHP 7.4, `$x = null; echo $x['y'];` does not emit a warning. Sneaky!

Test Plan:
- Traced `null` values from `reparseHunksForSpecialAttributes()`, saw them no longer incorporated into corpus bodies.
- This has some amount of test coverage.

Differential Revision: https://secure.phabricator.com/D21169