title: image_tag('#') throw error The asset "#" is not present in the asset pipeline
body: <h2>Steps to reproduce</h2>
  <ul>
  <li>Use <code>#</code> for <code>image_tag</code></li>
  </ul>
  <pre><code>image_tag '#', {}
  </code></pre>
  <h2>Expected behavior</h2>
  <p>Should it combine something like this <code>&lt;img src=&quot;#&quot;/&gt;</code></p>
  <h2>Actual behavior</h2>
  <p>Raise exception:</p>
  <pre><code>Sprockets::Rails::Helper::AssetNotFound
  The asset &quot;#&quot; is not present in the asset pipeline.
  </code></pre>
  <h3>System configuration</h3>
  <p>Rails version: 6.0.3</p>
  <p>Ruby version: 2.6.6</p>