activerecord: AR::Relation#new should take block argument as optional

[tk0miya]

Now the block argument for AR::Relation#new is defined as required, but it must be optional.

refs:

• https://api.rubyonrails.org/classes/ActiveRecord/Relation.html#method-i-new
• https://github.com/rails/rails/blob/v6.0.6.1/activerecord/lib/active_record/relation.rb#L69

[github-actions]

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activerecord

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[ksss]

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[github-actions]

Thanks for your review, @ksss!

@tk0miya, @ksss This PR is ready to be merged.
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[tk0miya]

/merge