Add constraint null checking in graph components

[hrntsm]

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Reference Issues/PRs

What does this implement/fix? Explain your changes.

In the same study, when a trial with constraints and a trial without constraints were mixed together, an exception was generated when drawing graphs.

Specifically, the null check was added because a Trial without constraints would be null.
In the case of null, Trial is feasible.

[porink0424]

Thank you for the PR!

Currently, type Trial.constraints is implemented in a way that null cannot be included (also, in the serialize_frozen_trial function of optuna_dashboard/_serializer.py, constraints are created like "constraints": trial_system_attrs.get(CONSTRAINTS_KEY, []), which means I don't think null will be included). So I'm not sure if it's really necessary to consider the possibility of null here.

If it is indeed necessary, could you provide an example code that produces such a situation?

[hrntsm]

@porink0424

Thank you for checking it out.
I was not able to reproduce the problem, so I reviewed the log file and found that it was caused by the following null included in the file.

{"op_code": 9, "worker_id": "5609884b-ac1d-4851-854b-c2baa6e293e4-5712", "trial_id": 5, "system_attr": {"constraints": null}}

However, looking at the following Optuna implementation, it seems that it would result in an error in case of null.
Therefore, this kind of result should not occur.
As I can't understand the cause, I think I will close this PR for now.

https://github.com/optuna/optuna/blob/8476b460fb4ad79e9051a3b7ac49629d0e389ac9/optuna/samplers/_base.py#L244