[Needs prioritisation] & More understanding of overall core issue.

Make the assumptions in the UsageDeduplicator code explicit, log an error if the assumption is broken, but handle gracefully (do not let it crash)

This need was brought to our attention by this production error T428620: TypeError: Wikibase UsageDeduplicator::deduplicateStatementUsages(): Argument must be of type array (Warning: Undefined array key "C") (☀️)

Todo:

• Check what the assumptions are (e.g. there should be no CQR usage without C usage)