Fix detection of null relations for Rails 7

What does this MR do and why?

In Rails 7, ActiveRecord::NullRelation.to_sql no longer results into a blank string. So we check for the class of the relation instead.

Rails 6:

[1] pry(main)> Issue.none.to_sql
=> ""

Rails 7:

[1] pry(main)> Issue.none.to_sql
=> "SELECT \"issues\".* FROM \"issues\" WHERE (1=0)"

Related failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/3669887028

Refs &7875 (closed)

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