CI Linter fails if there's no code yet

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1638758/?referrer=gitlab_plugin

NoMethodError: undefined method `sha' for nil:NilClass
  projects/ci/lints_controller.rb:32:in `yaml_processor_options'
    sha: project.repository.commit.sha
  projects/ci/lints_controller.rb:11:in `create'
    result   = Gitlab::Ci::YamlProcessor.new_with_validation_errors(@content, yaml_processor_options)
  action_controller/metal/basic_implicit_render.rb:6:in `send_action'
    super.tap { default_render unless performed? }
  abstract_controller/base.rb:195:in `process_action'
    send_action(method_name, *args)
  action_controller/metal/rendering.rb:30:in `process_action'
    super
...
(140 additional frame(s) were not displayed)

NoMethodError: undefined method `sha' for nil:NilClass