store null as default value againt Null boolean field

An empty string "" is not None

>>> "" is None
False

If you want the default to be None then write:

employed = models.NullBooleanField(choices=LOCATOR_YES_NO_CHOICES,
                                max_length=3,
                                blank=True, null=True, default=None,)

Note that NullBooleanFIeld was deprecated in Django 3.1. It is recommended to use BooleanField(null=True).

In this case;

employed = Models.BooleanField(null=True, default=None)