Fix an error when .rubocop.yml is empty

[koic]

This PR fixes the following error when .rubocop.yml is empty.

% touch .rubocop.yml

% rubocop
/Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/server/cache.rb:75:in
`block in cache_root_dir_from_config': undefined method `dig' for nil:NilClass (NoMethodError)

            config_yaml.dig('AllCops', 'CacheRootDirectory')
                       ^^^^
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/cache_config.rb:9:in
                       `root_dir'
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/server/cache.rb:61:in
                       `cache_root_dir_from_config'
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/server/cache.rb:54:in
                       `cache_path'
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/server/cache.rb:45:in
                       `dir'
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/lib/rubocop/server.rb:36:in
                       `running?'
        from /Users/koic/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rubocop-1.34.1/exe/rubocop:11:in
                       `<top (required)>'
        from /Users/koic/.rbenv/versions/3.1.2/bin/rubocop:25:in `load'
        from /Users/koic/.rbenv/versions/3.1.2/bin/rubocop:25:in `<main>'

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

FYI, this is to avoid the following incompatibility.

% ruby -ryaml -ve "p YAML.load('')"
ruby 3.0.4p208 (2022-04-12 revision 3fa771dded) [x86_64-darwin19]
false

% ruby -ryaml -ve "p YAML.load('')"
ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-darwin19]
nil

The added test will not pass the CI matrix if this statement doesn't exist.

[bbatsov]

Thanks!

[bbatsov]

Might also be a good idea to issue some warning when this happens as an empty .rubocop.yml doesn't make much sense in general.