title: Symfony form CollectionType remove empty value
body: <p>I'm using Symfony 3.3 and I have a form with a CollectionType like : </p>
  
  <pre><code>$builder-&gt;add('links', CollectionType::class, array(
      'label' =&gt; false,
      'entry_type' =&gt; LinkType::class,
      'entry_options' =&gt; ['data_class' =&gt; CompanyLink::class],
      'allow_add' =&gt; true,
      'allow_delete' =&gt; true,
      'prototype' =&gt; true,
      'by_reference' =&gt; false,
  ));
  </code></pre>
  
  <p>I followed the documentation : <a href="https://symfony.com/doc/current/reference/forms/types/collection.html" rel="nofollow noreferrer">https://symfony.com/doc/current/reference/forms/types/collection.html</a></p>
  
  <p>This is what I have in my view : </p>
  
  <p><a href="https://i.stack.imgur.com/wqlia.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wqlia.png" alt="enter image description here"></a></p>
  
  <p>So, the form displays well data already in my database (one line each time) but it always adds an empty field in addition.</p>
  
  <p>How can I remove this empty field? Because I want to have this line only I click on the button "Ajouter".</p>
  
  <p>Thanks!</p>