Fix an error for apply_rubocop_autocorrect_after_generate! with --pretend

## Motivation / Background

An issue was identified where an error occurs when executing `apply_rubocop_autocorrect_after_generate!` with `--pretend` option,
according to feedback from rubocop/rubocop-rails#1263.

## Details

This PR fixes the following error when executing `apply_rubocop_autocorrect_after_generate!` with `--pretend` option:

```console
$ bin/rails g migration create_users -p
```

### Before

An `Errno::ENOENT` error occurs:

```console
invoke  active_record
create    db/migrate/20240329060627_create_users.rb
/Users/koic/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/bundler/gems/rails-8e46af8c9396/railties/lib/rails/configuration.rb:138:in
`system': No such file or directory - bin/rubocop (Errno::ENOENT)
```

### After

No errors.

Author: koic

railties/lib/rails/configuration.rb

@@ -133,7 +133,7 @@ def after_generate(&block)
 
       def apply_rubocop_autocorrect_after_generate!
         after_generate do |files|
-          parsable_files = files.filter { |file| file.end_with?(".rb") }
+          parsable_files = files.filter { |file| File.exist?(file) && file.end_with?(".rb") }
           unless parsable_files.empty?
             system("bin/rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
           end

railties/test/application/generators_test.rb

@@ -265,6 +265,16 @@ def check_expected
         output = rails("generate", "model", "post", "title:string", "body:string")
         assert_match(/3 files inspected, no offenses detected/, output)
       end
+
+      test "generators with apply_rubocop_autocorrect_after_generate! and pretend" do
+        with_config do |c|
+          c.generators.apply_rubocop_autocorrect_after_generate!
+        end
+
+        assert_nothing_raised do
+          rails("generate", "model", "post", "title:string", "body:string", "--pretend")
+        end
+      end
     end
   end
 end