Use `:empty_project` where possible in request specs

spec/requests/api/branches_spec.rb

spec/requests/api/builds_spec.rb

spec/requests/api/commit_statuses_spec.rb

spec/requests/api/commits_spec.rb

spec/requests/api/files_spec.rb

spec/requests/api/fork_spec.rb

spec/requests/api/internal_spec.rb

spec/requests/api/merge_requests_spec.rb

spec/requests/api/pipelines_spec.rb

spec/requests/api/projects_spec.rb

spec/requests/api/repositories_spec.rb

spec/requests/api/tags_spec.rb

spec/requests/api/triggers_spec.rb

spec/requests/ci/api/triggers_spec.rb

spec/requests/git_http_spec.rb

spec/requests/projects/artifacts_controller_spec.rb

spec/requests/projects/cycle_analytics_events_spec.rb