Specify unset_nil: false to ConfigLoader.merge_with_default

[koic]

Follow up rubocop/rubocop#7048.

This PR fixes the following warning.

% cd path/to/rubocop-performance
% bundle exec rake generate_cops_documentation
Files:          54
Modules:         4 (    2 undocumented)
Classes:        55 (    0 undocumented)
Constants:     122 (  122 undocumented)
Attributes:      1 (    0 undocumented)
Methods:       117 (  100 undocumented)
 25.08% documented
Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion

Warning: Rails/BulkChangeTable does not support Database parameter.

Supported parameters are:

  - Enabled
  - SupportedDatabases
  - Include

* generated
  /Users/koic/src/github.com/rubocop-hq/rubocop-rails/manual/cops_rails.md

Note: Use of unset_nil: false option requires a release higher than RuboCop 0.69.0.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.