Three bugs:
- The parser wrapping was generating hashes for fallback values in absence of actual values for both URL and title (which is valid if obscure RSS), URL and content, or title and content; now fallback values are detected and empty strings used instead of hashes
- The deduplicator was assuming all three hashes would always exist, which is no longer the case
- The database matcher was making the same assumption as the deduplicator
