Use `:empty_project` where possible in service specs

spec/services/after_branch_delete_service_spec.rb

spec/services/ci/create_pipeline_service_spec.rb

spec/services/ci/create_trigger_request_service_spec.rb

spec/services/ci/stop_environments_service_spec.rb

spec/services/ci/update_build_queue_service_spec.rb

spec/services/compare_service_spec.rb

spec/services/create_release_service_spec.rb

spec/services/delete_branch_service_spec.rb

spec/services/delete_merged_branches_service_spec.rb

spec/services/files/update_service_spec.rb

spec/services/git_hooks_service_spec.rb

spec/services/git_push_service_spec.rb

spec/services/git_tag_push_service_spec.rb

spec/services/groups/destroy_service_spec.rb

spec/services/groups/update_service_spec.rb

spec/services/issues/build_service_spec.rb

spec/services/issues/move_service_spec.rb

spec/services/issues/resolve_discussions_spec.rb

spec/services/labels/find_or_create_service_spec.rb

spec/services/labels/transfer_service_spec.rb