Handle delete-only case in zBuilder.sh

[lauren-li]

Similar to issue #305 but for zBuilder: If a developer just commits a change deleting a file (and nothing else), the Common Back-end Scripts (CBS) will set the Return Code of the Build stage to 4 and stop the pipeline with a "No source changes detected" error, rather than continuing the pipeline run (e.g. with packaging of the deletions). This may have to do with line 523 of zBuilder.sh.