title: nullable environment variables in Symfony
body: <p>I know that env variables are string only. Symfony 3.4 <a href="https://symfony.com/blog/new-in-symfony-3-4-advanced-environment-variables" rel="noreferrer">supports env variables type casting</a>.
  Is there a way to pass null value through int validator?</p>
  
  <p>.env</p>
  
  <pre><code>DATABASE_PORT=
  #I also tried DATABASE_PORT=null, DATABASE_PORT=~
  </code></pre>
  
  <p>parameters.yml</p>
  
  <pre><code>app.connection.port: '%env(int:DATABASE_PORT)%'
  #I also tried env(?int, env(int?
  </code></pre>
  
  <p>I'm getting an error: "Non-numeric env var "DATABASE_PORT" cannot be cast to int." or "Invalid env(?int:DATABASE_PORT) name: only "word" characters are allowed."</p>
  
  <p>In yml there are ~ or null signs used to pass null, e.g:</p>
  
  <pre><code>app.connection.port: ~
  </code></pre>