Unset a reference field of an existing record triggers a 400 - Bad request

[bakulf]

If there is class with an optional ManyToOne property, setting the corresponding field of an existing record to null/0/empty returns a 400 - Bad Request, with error message: ""Expected IRI or nested document for attribute \u0022foobar\u0022, \u0022string\u0022 given."

This happens because the admin panel sends an empty string instead of a null value.

STR:

1. 2 classes:

class A {
  /**
   * @ORM\Id
   * @ORM\GeneratedValue
   * @ORM\Column(type="integer")
   */
  private $id; 
  /**
   * @ORM\ManyToOne(targetEntity="B", inversedBy="foos")
   */
    public $foobar; 
    public function getId(): int { return $this->id; }
}
class B
{
  /**
   * @ORM\Id
   * @ORM\GeneratedValue
   * @ORM\Column(type="integer")
   */                                                                                                          
   private $id;                                                                                                             
   /**
    * @ORM\OneToMany(targetEntity="A", mappedBy="foobar")
    */                                                                                                          
   public $foos;                                                                                                              
   public function __construct() { $this->foos = new ArrayCollection(); }                                                                                                           
   public function getId(): int { return $this->id; }                                       
}

2. create a record in B:
   curl 'http://localhost:8000/api/bs' --data '{}' -H 'content-type: application/ld+json'
3. create a record in A referring to the B's record:
   curl 'http://localhost:8000/api/as' --data '{"foobar":"/api/bs/XXX"}' -H 'content-type: application/ld+json'
4. update A's record from the admin panel:
   Using the admin, deselect the field 'foobar' of the record /api/as/YYY and submit.

Expected results:
the A's record should have an empty 'foobar' relationship with class B.

Actual results:
400 Bad Request.

I suspect that the bug is in convertReactAdminDataToHydraData:
https://github.com/api-platform/admin/blob/master/src/hydra/hydraClient.js#L123-L128

We should normalize the empty string value to a null.
Here what I wrote to fix the issue locally:

    resource.fields.forEach(function (_ref2) {
      var name = _ref2.name,
          normalizeData = _ref2.normalizeData;

      if (!(name in data)) {
        return;
      }

      if (_ref2.reference && data[name] === "") {
        data[name] = null;
      }

      if (undefined === normalizeData) {
        return;
      }

      fieldData[name] = normalizeData(data[name]);
    });