title: Why am I getting "Unexpected value ''" error in this GitHub action?
body: <p>I have a GitHub action step like this (extracted from a larger <code>test.yml</code> file):</p>
  <pre class="lang-yaml prettyprint-override"><code>    steps:
      - name: Parse
        shell: bash
        env:
          TYPE: ${{matrix.package-type}}
          BV: ${{matrix.builder-version}}
          # This comment is line 63, the &quot;#&quot; is in column 9
          NULL: ${{ matrix.beta-version }}
        run: |
          echo TYPE is &quot;$TYPE&quot;
          echo BV is &quot;$BV&quot;
          printf &quot;Null is '%s'\n&quot; &quot;$NULL&quot;
  </code></pre>
  <p>When I run it, I get the following error:</p>
  <pre><code>The workflow is not valid. .github/workflows/test.yml (Line: 64, Col: 9): Unexpected value ''
  </code></pre>
  <p>Why is this line invalid? How do I fix it?</p>