fix: raise ValueError when retrieval evaluators get no questions

[Vamsi-2581]

What happened

DocumentMAPEvaluator, DocumentMRREvaluator, DocumentRecallEvaluator, and AnswerExactMatchEvaluator average per-question scores with sum(scores) / len(questions). An empty question list (a filtered eval set, for example) therefore raised ZeroDivisionError instead of a useful error. DocumentNDCGEvaluator already rejects that input with ValueError.

What changed

Each of those four evaluators now raises ValueError when either input list is empty, before the mean is computed. A question whose own document list is empty still scores 0.0, which is the existing per-question behavior.

How it was checked

New tests in test/components/evaluators/test_empty_input.py cover both lists empty, one list empty, and a non-empty question with no documents. The empty cases raise ValueError (must be provided) instead of dividing by zero.

python -m pytest test/components/evaluators/test_empty_input.py test/components/evaluators/test_document_map.py test/components/evaluators/test_document_mrr.py test/components/evaluators/test_document_recall.py test/components/evaluators/test_answer_exact_match.py test/components/evaluators/test_document_ndcg.py -q
98 passed

Fixes #12543

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