Fix an error for Style/Copyright

This PR fixes the following error for `Style/Copyright` when `AutocorrectNotice` is missing:

```console
$ cat .rubocop.yml
Style/Copyright:
  Enabled: true
  AutocorrectNotice: ~

$ bundle exec rubocop -d
(snip)

undefined method 'empty?' for nil
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/copyright.rb:61:
in 'RuboCop::Cop::Style::Copyright#verify_autocorrect_notice!'
```

changelog/fix_an_error_for_style_copyright.md

@@ -0,0 +1 @@
+* [#12951](https://github.com/rubocop/rubocop/pull/12951): Fix an error for `Style/Copyright` when `AutocorrectNotice` is missing. ([@koic][])

lib/rubocop/cop/style/copyright.rb

@@ -58,7 +58,9 @@ def autocorrect_notice
         end
 
         def verify_autocorrect_notice!
-          raise Warning, "#{cop_name}: #{AUTOCORRECT_EMPTY_WARNING}" if autocorrect_notice.empty?
+          if autocorrect_notice.nil? || autocorrect_notice.empty?
+            raise Warning, "#{cop_name}: #{AUTOCORRECT_EMPTY_WARNING}"
+          end
 
           regex = Regexp.new(notice)
           return if autocorrect_notice.gsub(/^# */, '').match?(regex)

spec/rubocop/cop/style/copyright_spec.rb

@@ -114,7 +114,7 @@ class Foo
     end
 
     it 'fails to autocorrect if no AutocorrectNotice is given' do
-      # cop_config['AutocorrectNotice'] = '# Copyleft (c) 2015 Acme Inc.'
+      cop_config['AutocorrectNotice'] = nil
       expect { expect_offense(source) }.to raise_error(RuboCop::Warning, %r{Style/Copyright:})
     end
   end