Generalize lemma null_set_integral

[Yosuke-Ito-345]

Motivation for this change

I noticed that we could slightly generalize the lemma null_set_integral.
The generalized version is named null_set_integral_gen.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md
• added corresponding documentation in the headers

Reference: How to document

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[Yosuke-Ito-345]

@affeldt-aist @t6s
It's a small generalization, but I'd like to use them for my formalization of actuarial mathematics.
I kindly ask for your review.

[affeldt-aist]

It is maybe better to replace the old lemma by its generalization rather than introduce a new lemma (I did that in the second to last commit).

[Yosuke-Ito-345]

@affeldt-aist
Thank you for the review!
I agree with replacing the old lemma.