This commit addresses the `DEPRECATED: Use assert_nil if expecting nil` warning at
https://buildkite.com/rails/rails/builds/116902#01953bd2-7999-468a-bcd6-5e9b68964fa6/1270-1276
by enabling `Minitest/AssertNil` available since RuboCop Minitest 0.1.0.

```ruby
$ cd railties
$ RAILS_STRICT_WARNINGS=1 bin/test test/application/configuration_test.rb:4031
Run options: --seed 1261

DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/src/github.com/rails/rails/railties/test/application/configuration_test.rb:4031. This will fail in Minitest 6.
E

Error:
ApplicationTests::ConfigurationTest#test_ActiveStorage.variant_processor_uses_mini_magick_without_Rails_7_defaults:
ActiveSupport::RaiseWarnings::WarningError: DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/src/github.com/rails/rails/railties/test/application/configuration_test.rb:4031. This will fail in Minitest 6.

    /home/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/testing/strict_warnings.rb:38:in 'ActiveSupport::RaiseWarnings#warn'

bin/test test/application/configuration_test.rb:4026

Finished in 1.568723s, 0.6375 runs/s, 0.6375 assertions/s.
1 runs, 1 assertions, 0 failures, 1 errors, 0 skips
$
```

Here are the output of rubocop -a that corrects other assertions that are not deprecated.

```ruby
$ bundle exec rubocop -a
... snip ...
Offenses:

actionpack/test/controller/integration_test.rb:498:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(request.parameters["leaks"]).
      assert_predicate request.parameters["leaks"], :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
actiontext/test/unit/model_test.rb:21:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(message.content).
      assert_predicate message.content, :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
actiontext/test/unit/model_test.rb:149:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(message.content).
      assert_predicate message.content, :nil?
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/type/boolean_test.rb:10:9: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(type.cast("")).
        assert_predicate type.cast(""), :nil?
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/type/boolean_test.rb:11:9: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(type.cast(nil)).
        assert_predicate type.cast(nil), :nil?
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activemodel/test/cases/validations_test.rb:255:5: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(t.author_name).
    assert_predicate t.author_name, :nil?
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
activerecord/test/cases/associations/has_many_through_associations_test.rb:1246:5: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(post[:author_count]).
    assert_predicate post[:author_count], :nil?
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
railties/test/application/configuration_test.rb:4031:7: C: [Corrected] Minitest/AssertNil: Prefer using assert_nil(ActiveStorage.variant_processor).
      assert_equal nil, ActiveStorage.variant_processor
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

3345 files inspected, 8 offenses detected, 8 offenses corrected
$
```

Refer to
https://github.com/rubocop/rubocop-minitest/blob/master/CHANGELOG.md#010-2019-09-01
https://github.com/minitest/minitest/blob/master/History.rdoc#5230--2024-05-15-
https://github.com/minitest/minitest/commit/f0f17b99