title: Symfony Date Constraint does not allow missing field even though the field is not required
body: <p>I am instantiating a constraint collection here to validate my request body for an API I am building. My idea is to validate the <em>birthdate</em> param only for its format, I don't need it to be required. The issue I am having is that, when I do not pass the <em>birthdate</em> in the request body, It throws a missing field error. Basically it treats it as a required field.
  I don't know why.</p>
  <pre><code>$constraint = new Collection([
    'fields' =&gt; [
      'birthdate' =&gt; [
        new Date(message: 'Please use YYYY-MM-DD format!'),
      ],
    ],
    'allowMissingFields' =&gt; false,
    'allowExtraFields' =&gt; true,
  ]);
  </code></pre>