Bugfix: also schedule time based integration when source is 0 (#133438)

* Bugfix also schedule time based integration when source is 0

* Update tests/components/integration/test_sensor.py

Co-authored-by: Diogo Gomes <diogogomes@gmail.com>

* Improve comment in test. Remove redundant assertion.

---------

Co-authored-by: Diogo Gomes <diogogomes@gmail.com>

homeassistant/components/integration/sensor.py

@@ -576,7 +576,7 @@ class IntegrationSensor(RestoreSensor):
         if (
             self._max_sub_interval is not None
             and source_state is not None
-            and (source_state_dec := _decimal_state(source_state.state))
+            and (source_state_dec := _decimal_state(source_state.state)) is not None
         ):
 
             @callback

tests/components/integration/test_sensor.py

@@ -843,6 +843,39 @@ async def test_on_valid_source_expect_update_on_time(
         assert float(state.state) < 1.8
 
 
+async def test_on_0_source_expect_0_and_update_when_source_gets_positive(
+    hass: HomeAssistant,
+) -> None:
+    """Test whether time based integration updates the integral on a valid zero source."""
+    start_time = dt_util.utcnow()
+
+    with freeze_time(start_time) as freezer:
+        await _setup_integral_sensor(hass, max_sub_interval=DEFAULT_MAX_SUB_INTERVAL)
+        await _update_source_sensor(hass, 0)
+        await hass.async_block_till_done()
+
+        # wait one minute and one second
+        freezer.tick(61)
+        async_fire_time_changed(hass, dt_util.now())
+        await hass.async_block_till_done()
+
+        state = hass.states.get("sensor.integration")
+
+        assert condition.async_numeric_state(hass, state) is True
+        assert float(state.state) == 0  # integral is 0 after integration of 0
+
+        # wait one second and update state
+        freezer.tick(1)
+        async_fire_time_changed(hass, dt_util.now())
+        await _update_source_sensor(hass, 100)
+        await hass.async_block_till_done()
+
+        state = hass.states.get("sensor.integration")
+
+        # approx 100*1/3600 (right method after 1 second since last integration)
+        assert 0.027 < float(state.state) < 0.029
+
+
 async def test_on_unvailable_source_expect_no_update_on_time(
     hass: HomeAssistant,
 ) -> None: