title: Django rest framework empty field in partial update not updated
body: <p>When I try to do a partial update with patch or a update with put. The object first looks like this:</p>
  
  <pre><code>{
    "name":"Amsterdam 1",
    "location":"Amsterdam",
    "client":"b9c7d1c9-4b1b-4f0d-af30-2d8ffb97647e"
  }
  </code></pre>
  
  <p>Then I remove the <code>client</code> on the object and thus send this back:</p>
  
  <pre><code>{
    "name":"Amsterdam 1",
    "location":"Amsterdam"
  }
  </code></pre>
  
  <p>But this does not remove the client from the object.</p>
  
  <p>The model looks like this:</p>
  
  <pre><code>
  class FixedLocation(models.Model):
      id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
      name = models.CharField(max_length=155)
      client = models.ForeignKey(Client, on_delete=models.CASCADE, null=True, blank=True)
      location = models.CharField(max_length=263)
  </code></pre>
  
  <p>And this is how my serializer looks</p>
  
  <pre><code>class FixedLocationSerializer(serializers.ModelSerializer):
      class Meta:
          model = models.FixedLocation
          fields = ["id", "name", "location", "client"]
  </code></pre>
  
  <p>I expect when I don't send <code>client</code> back that it gets removed from the object. Does anybody know how I can achieve this?</p>
  
  <p>I also tried playing with <code>null</code> and <code>blank</code> on the foreign key.</p>