Fix an issue where Herald may fail to extract content from an empty commit

Summary:
Ref T13667. The Herald "content added" rule (and other similar rules) do not correctly extract content from empty commits.

When we load an empty raw diff, return an empty changed content map.

Ref T13588. Also fix some PHP8.1 null/string stuff

Test Plan:

• Ran "bin/repository reparse --publish <commit>", with an empty commit hash and a nonempty commit hash.
• Reviewed Herald transcripts for general sanity.

Maniphest Tasks: T13667, T13588

Differential Revision: https://secure.phabricator.com/D21761