[Fix rubocop#8423] Fix an error for Style/SingleArgumentDig

Fixes rubocop#8423.

This PR fixes an error for `Style/SingleArgumentDig`
when without a receiver.
This cop wouldn't have to offense when `dig` method without a receiver.

CHANGELOG.md

@@ -30,6 +30,7 @@
 * [#8406](https://github.com/rubocop-hq/rubocop/issues/8406): Improve `Style/AccessorGrouping`'s auto-correction to remove redundant blank lines. ([@koic][])
 * [#8330](https://github.com/rubocop-hq/rubocop/issues/8330): Fix a false positive for `Style/MissingRespondToMissing` when defined method with inline access modifier. ([@koic][])
 * [#8422](https://github.com/rubocop-hq/rubocop/issues/8422): Fix an error for `Lint/SelfAssignment` when using or-assignment for constant. ([@koic][])
+* [#8423](https://github.com/rubocop-hq/rubocop/issues/8423): Fix an error for `Style/SingleArgumentDig` when without a receiver. ([@koic][])
 
 ### Changes
 

lib/rubocop/cop/style/single_argument_dig.rb

@@ -33,6 +33,8 @@ class SingleArgumentDig < Base
         PATTERN
 
         def on_send(node)
+          return unless node.receiver
+
           expression = single_argument_dig?(node)
           return unless expression
 

spec/rubocop/cop/style/single_argument_dig_spec.rb

@@ -66,4 +66,12 @@
       end
     end
   end
+
+  context 'when without a receiver' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        dig(:key)
+      RUBY
+    end
+  end
 end