Confidence scores should be nullable in the SearchCorpus API docs

While working on frontend#1135 (closed), I found that transcription_confidence and classification_confidence are marked as optional (they can be entirely missing from the API response) and non-nullable, when they actually are always present (we almost never remove fields entirely from serializers) and can be null when there is no transcription or classification linked to this search result, or when the confidence of the related transcription or classification is null.