message: fix: use -z flag for var unset in canary
diff: @@ -74,11 +74,11 @@ jobs:
  - name: Docker Image Name
  id: docker_image_name
  run: |
  - if [ $CUSTOM_IMAGE_NAME == '' ]
  + if [ -z "${CUSTOM_IMAGE_NAME}" ]
  then
  echo "::set-output name=canary-docker-tag::${GITHUB_SHA::8}"
  else
  - echo "::set-output name=canary-docker-tag::prerelease-$CUSTOM_IMAGE_NAME"
  + echo "::set-output name=canary-docker-tag::prerelease-${CUSTOM_IMAGE_NAME}"
  fi
  env:
  CUSTOM_IMAGE_NAME: ${{ github.event.inputs.customImageName }}
  @@ -112,7 +112,7 @@ jobs:
  context: .
  file: ./ops/docker/Dockerfile.geth
  push: true
  - tags: ethereumoptimism/l2geth:${{ needs.canary-publish.outputs.l2geth }}
  + tags: ethereumoptimism/l2geth:${{ steps.docker_image_name.outputs.canary-docker-tag }}
  # pushes the base builder image to dockerhub
  builder:
  