title: Symfony asset component "" is not a valid URL
body: <p>I am using webpack dev server to serve my assets in local development. I have following in my paramters.yml.dist</p>
  
  <pre><code># local assets devserver address
  # if you want to use webpack dev server use "http://localhost:8090"
  # and then &gt; npm run devserver
  assets_base_url: ~
  </code></pre>
  
  <p>I have following in framework for local environment.</p>
  
  <pre><code>framework:
      assets:
          base_urls: ["%assets_base_url%"]
  </code></pre>
  
  <p>My problem is that it is not working with base settings of "~" for my colleagues that are not using devserver.</p>
  
  <pre><code>Error: "" is not a valid URL
  </code></pre>
  
  <p>What should I do enable or disable devserver assets url by one setting in parameters.yml </p>
  
  <p><a href="https://symfony.com/blog/new-in-symfony-2-7-the-new-asset-component" rel="nofollow">Symfony docs for asset component</a></p>