Fix cop for classes inside migrations

What does this MR do and why?

This fixes an error that appears when a migration contains a class that does not have a parent class. I found this error working on this migration: !122850 (diffs)

How to set up and validate locally

1. On the master branch.
2. Create a migration rails g migration test_rubocop_error.
3. Create a class inside the migration.

     class TestRubocopError < Gitlab::Database::Migration[2.1]
       class TestClass
       end
   
       def change
       end
     end

4. Run rubocop with your migration file rubocop db/migrate/20230815144647_test_rubocop_error.rb.
5. Observe an error An error occurred while Migration/VersionedMigrationClass cop was inspecting....
6. Switch to the andysoiron/fix-cop-for-nested-classes-in-migrations branch.
7. Run rubocop again rubocop db/migrate/20230815144647_test_rubocop_error.rb.
8. Observe a rubocop failure Lint/EmptyClass: Empty class detected.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.