Change STATE_UNKOWN to None (#20337)

* Change STATE_UNKOWN to None

* Change STATE_UNKOWN to None

* tests

* tests

* tests

* tests

* tests

* style

* fix comments

* fix comments

* update fan test

homeassistant/components/sensor/tank_utility.py

@@ -13,8 +13,7 @@ import voluptuous as vol
 
 import homeassistant.helpers.config_validation as cv
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_DEVICES, CONF_EMAIL, CONF_PASSWORD,
-                                 STATE_UNKNOWN)
+from homeassistant.const import CONF_DEVICES, CONF_EMAIL, CONF_PASSWORD
 from homeassistant.helpers.entity import Entity
 
 
@@ -78,7 +77,7 @@ class TankUtilitySensor(Entity):
         self._password = password
         self._token = token
         self._device = device
-        self._state = STATE_UNKNOWN
+        self._state = None
         self._name = "Tank Utility " + self.device
         self._unit_of_measurement = SENSOR_UNIT_OF_MEASUREMENT
         self._attributes = {}