fix(prepro): correctly handle empty ndjson response

[corneliusroemer]

Avoids this error when no data available:

DEBUG:urllib3.connectionpool:http://loculus-backend-service:8079 "POST /cchf/extract-unprocessed-data HTTP/11" 200 610
DEBUG:charset_normalizer:Encoding detection: ascii is most likely the one.
Traceback (most recent call last):
  File "/opt/conda/bin/prepro", line 8, in <module>
    sys.exit(cli_entry())
             ^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/site-packages/loculus_preprocessing/__main__.py", line 16, in cli_entry
    run(config)
  File "/opt/conda/lib/python3.12/site-packages/loculus_preprocessing/prepro.py", line 748, in run
    etag, unprocessed = fetch_unprocessed_sequences(etag, config)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/site-packages/loculus_preprocessing/backend.py", line 110, in fetch_unprocessed_sequences
    return response.headers["ETag"], parse_ndjson(response.text)
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/site-packages/loculus_preprocessing/backend.py", line 78, in parse_ndjson
    json_object = json.loads(json_str_processed)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

[corneliusroemer]

Actually the error was because the backend errored:

Exception: Failed to parse JSON: An unexpected error occurred while streaming, aborting the stream: java.sql.BatchUpdateException: Batch entry 0 INSERT INTO sequence_entries_preprocessed_data (accession, pipeline_version, processing_status, started_processing_at, “version”) VALUES ((‘PP_000KBM9’), (‘2’::int8), (‘IN_PROCESSING’), (‘2024-10-24 21:49:16.448672+00’), (‘2’::int8)) was aborted: ERROR: duplicate key value violates unique constraint “sequence_entries_preprocessed_data_pkey”

So we should catch and reraise with the full error message instead which I've added to:

• feat(prepro, ingest, deposition): Enforce author formatting in prepro, map authors accordingly in ingest and deposition #2986