fix: evaluation docstring examples to use value=0 instead of value=None (#1520)

docs: fix Evaluation docstring examples to use value=0 instead of value=None

Update docstring examples in EvaluatorFunction and RunEvaluatorFunction
protocols to use value=0 instead of value=None, matching the type definition
which requires Union[int, float, str, bool] and does not allow None.

This change aligns with commit d11155e5492b401db227d87e32b587ad31bab7e9
which established that Evaluation score values cannot be None.

Changes:
- Update accuracy_evaluator example in EvaluatorFunction docstring
- Update llm_judge_evaluator error handling example
- Update average_accuracy example in RunEvaluatorFunction docstring
- Update accuracy_evaluator example in DatasetClient.run_experiment docstring