Unify the behavior of the terminator and its visualization function

[HideakiImamura]

Motivation

We are developing optuna.terminator as an automatic termination module, and have prepared optuna.visualization.plot_terminator_improvement for its visualization. However, there are inconsistencies in their behavior, and I propose that we align them.

Description

The optuna.terminator.Terminator class is initialized with improvement_evaluator, error_evaluator, and min_n_trials. If improvement_evaluator is an instance of BestValueStagnationEvaluator and error_evaluator is None, error_evaluator is initialized with StaticErrorEvaluator(constant=0) (ref). plot_terminator_improvement is not set up like this. The latter should be made to match the former.

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