Pipelines errors if variable for service is empty

image: circleci/ruby:2.6.3-node-browsers

pipelines:
  default:
    - step:          # non-parallel step
        name: Build
        script:
          - ./.ci/build.sh
        caches:
          - vendor-bundle
          - yarn-cache
          - node-modules
          - cache-cypress
        artifacts:
          - node_modules/**
          - vendor/bundle/**
    - parallel:
        - step:
            name: Rspec
            services:
              - postgres
            script:
              - ./.ci/rspec.sh
        - step:
            name: Cypress
            script:
              - ./.ci/cypress.sh
        - step:
            name: Jest
            script:
              - ./.ci/jest.sh
        - step:
            name: Static Analyisis
            script:
              - ./.ci/static_analysis.sh

definitions:
  services:
    postgres:
      image: postgres
      variables:
        POSTGRES_USER: nexl_db_user
        POSTGRES_DB: nexl_test
        POSTGRES_PASSWORD: ""
  caches:
    vendor-bundle: vendor/bundle
    yarn-cache: ~/.cache/yarn
    node-modules: node_modules
    cache-cypress: ~/.cache/Cypress