Temporary fix for causal trees missing values support #733

[alexander-pv]

Proposed changes

This PR contains a temporary stub aimed at fixing current causal trees issue #733 with new scikit-learn versions.

Related sklearn PRs:

• ENH Adds missing value support for trees scikit-learn/scikit-learn#23595
• ENH Adds support for missing values in Random Forest scikit-learn/scikit-learn#26391

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[jeongyoonlee]

Thanks for the quick fix, @alexander-pv. Can you fix the lint check failure by running the latest black?

[alexander-pv]

Thanks for the quick fix, @alexander-pv. Can you fix the lint check failure by running the latest black?

Done

[jeongyoonlee]

LGTM