Implement worker

Closes #2 (closed)

worker_generic_training_dataset/db.py

@@ -81,9 +81,11 @@ def retrieve_entities(transcription: CachedTranscription):
         .join(Entity, on=TranscriptionEntity.entity)
         .join(EntityType, on=Entity.type)
     )
-    return zip(
-        *[
-            parse_entities(entity_data, transcription)
-            for entity_data in query.namedtuples()
-        ]
-    )
+    data = [
+        parse_entities(entity_data, transcription)
+        for entity_data in query.namedtuples()
+    ]
+    if not data:
+        return [], []
+
+    return zip(*data)