Make classification confidence non nullable

arkindex/documents/export/structure.sql

...uments/migrations/0011_alter_classification_confidence.py

arkindex/documents/models.py

arkindex/documents/tests/tasks/test_corpus_delete.py

arkindex/documents/tests/test_classification.py

arkindex/documents/tests/test_retrieve_elements.py

arkindex/process/tests/test_managers.py

arkindex/process/tests/test_processes.py