Fixed bug which prevented to create a Computing with no user from the Django admin.

class Computing(models.Model):

    uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)

    user = models.ForeignKey(User, related_name='+', on_delete=models.CASCADE, null=True)

    user = models.ForeignKey(User, related_name='+', on_delete=models.CASCADE, blank=True, null=True)

    # If a compute resource has no user, it will be available to anyone. Can be created, edited and deleted only by admins.

    name = models.CharField('Computing Name', max_length=255, blank=False, null=False)