Errors in creations_controller.rb when attempting to create Merge Requests

Steps to reproduce

1. Create an issue
2. Click Create merge request button
3. It fails

Error 500 when creating merge requests with nil compare_commits

https://sentry.gitlap.com/gitlab/gitlabcom/issues/37315/

NoMethodError: undefined method `reverse' for nil:NilClass
  app/controllers/projects/merge_requests/creations_controller.rb:112:in `define_new_vars'
    @commits = @merge_request.compare_commits.reverse
  app/controllers/ee/projects/merge_requests/creations_controller.rb:10:in `define_new_vars'
    super
  app/controllers/projects/merge_requests/creations_controller.rb:27:in `create'
    define_new_vars
  lib/gitlab/i18n.rb:42:in `with_locale'
    yield
  lib/gitlab/i18n.rb:48:in `with_user_locale'
    with_locale(user&.preferred_language, &block)
...
(115 additional frame(s) were not displayed)