title: Allowing NULL value in json with API-Platform
body: <p>I have currently this entity and I want to show my property "firedDate" in my JSON even is the value is null.</p>
  
  <pre><code>/**
   * @ApiResource(normalizationContext={"groups"={"employee"}})
   * @ApiFilter(DateFilter::class, properties={"dateProperty": DateFilter::INCLUDE_NULL_BEFORE_AND_AFTER})
   * @ORM\Table(name="employee")
   */
  class Employee
  {
      // ...
  
      /**
       * @ORM\Column(type="datetime", nullable=true)
       * @Groups({"employee"})
       */
      private $firedDate;
  
  
      public function getFiredDate(): ?\DateTimeInterface
      {
          return $this-&gt;firedDate;
      }
  
      // ...
  }
  </code></pre>
  
  <p>What i'm doing wrong? :/
  Thanks!</p>