Allow fallback if undefined

[skjnldsv]

@nickvergessen

Should allow us to specify fallback values

name: Get node and npm versions from package.json

on: push

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v2

      - name: Read node and npm versions from package.json
        uses: skjnldsv/read-package-engines-version-actions@v1
        with: 
          fallbackNode: '^12'
          fallbackNpm: '^6'
        id: package-engines-versions

        ...