Don't have the ModelSerializer trust deserialized objects to not have redefine bool()ean-ness.

If the model we're using the ModelSerializer for has redefined methods that act as a boolean (__bool__ or __len__), it may not return the object even though it is_valid(), and should.

rest_framework/serializers.py

@@ -741,7 +741,7 @@ class ModelSerializer(Serializer):
         Override the default method to also include model field validation.
         """
         instance = super(ModelSerializer, self).from_native(data, files)
-        if instance:
+        if not self._errors:
             return self.full_clean(instance)
 
     def save_object(self, obj, **kwargs):