Fix an error when .rubocop.yml is empty

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

```console
% 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>'
```

changelog/fix_an_error_when_dot_rubocop_yml_is_empty.md

@@ -0,0 +1 @@
+* [#10916](https://github.com/rubocop/rubocop/pull/10916): Fix an error when .rubocop.yml is empty. ([@koic][])

lib/rubocop/server/cache.rb

@@ -69,10 +69,12 @@ def cache_root_dir_from_config
             config_yaml = if Gem::Version.new(Psych::VERSION) >= Gem::Version.new('4.0.0')
                             YAML.safe_load_file(config_path, permitted_classes: [Regexp, Symbol])
                           else
-                            YAML.load_file(config_path)
+                            config = YAML.load_file(config_path)
+
+                            config == false ? nil : config
                           end
 
-            config_yaml.dig('AllCops', 'CacheRootDirectory')
+            config_yaml&.dig('AllCops', 'CacheRootDirectory')
           end
         end
 

spec/rubocop/server/cache_spec.rb

@@ -205,5 +205,19 @@
         end
       end
     end
+
+    context 'when .rubocop.yml is empty', :isolated_environment do
+      context 'when cache root path is not specified path' do
+        before do
+          cache_class.cache_root_path = nil
+        end
+
+        it 'does not raise an error' do
+          create_file('.rubocop.yml', '')
+
+          expect { cache_class.cache_path }.not_to raise_error
+        end
+      end
+    end
   end
 end