Check if attributes are present in new_state before accessing them (#71967)

* Check if attributes are present in new_state before accessing them.

* Early return if new state is None|Unknown|Unavailable

* Removed whitespace at line endings. +black run

* Update test for coverage

tests/components/integration/test_sensor.py

@@ -9,6 +9,7 @@ from homeassistant.const import (
     ENERGY_WATT_HOUR,
     POWER_KILO_WATT,
     POWER_WATT,
+    STATE_UNAVAILABLE,
     STATE_UNKNOWN,
     TIME_SECONDS,
 )
@@ -350,6 +351,15 @@ async def test_units(hass):
     # they became valid
     assert state.attributes.get("unit_of_measurement") == ENERGY_WATT_HOUR
 
+    # When source state goes to None / Unknown, expect an early exit without
+    # changes to the state or unit_of_measurement
+    hass.states.async_set(entity_id, STATE_UNAVAILABLE, None)
+    await hass.async_block_till_done()
+
+    new_state = hass.states.get("sensor.integration")
+    assert state == new_state
+    assert state.attributes.get("unit_of_measurement") == ENERGY_WATT_HOUR
+
 
 async def test_device_class(hass):
     """Test integration sensor units using a power source."""