fix: preserve zero scores in document joiner

[wolfkill]

Summary

• fix DocumentJoiner duplicate selection in concatenate mode so score=0.0 is compared as a real score
• align merge-mode score handling with the same explicit None check
• add a regression test and release note

Why

DocumentJoiner._concatenate used a truthiness check for scores. Since 0.0 is falsy in Python, a duplicate document with score=0.0 was treated like an unscored document and could lose to a worse negative score.

Testing

• hatch -e test run pytest test/components/joiners/test_document_joiner.py::TestDocumentJoiner::test_run_with_concatenate_join_mode_keeps_duplicate_document_with_zero_score -q
• hatch -e test run pytest test/components/joiners/test_document_joiner.py -q
• env -u ALL_PROXY -u HTTPS_PROXY -u HTTP_PROXY -u all_proxy -u https_proxy -u http_proxy hatch -e test run pytest test/components/joiners -q
• hatch run fmt-check haystack/components/joiners/document_joiner.py test/components/joiners/test_document_joiner.py
• hatch run test:types haystack/components/joiners/document_joiner.py
• hatch -e test run python -m compileall -q haystack/components/joiners/document_joiner.py test/components/joiners/test_document_joiner.py
• git diff --check

Closes #11352

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[anakin87]

This is a duplicate of #11353 and the CLA is not signed.
For these reasons, I won't review it for the moment.