Remove empty human Subjects at the end of the Commit job

[janehmueller]

When a relation is edited in the Curation interface, the destination Subject is also marked as changed and sent in the Commit data even if there are no changes in it. This results in an empty human Subject in the knowledge base (all data fields are null). This can be prevented by removing all empty human Subjects before writing the curated Subjects to the Cassandra.