feat: Add availability property to DrivingAnalysisSensor

The `DrivingAnalysisSensor` now includes an `available` property to indicate the sensor's availability. This property returns `True` if the data for driving analysis is available, and `False` otherwise. This change improves the overall functionality of the sensor.

custom_components/nissan_carwings/sensor.py

@@ -182,6 +182,11 @@ def native_value(self) -> float | None:
         da: pycarwings3.responses.CarwingsDrivingAnalysisResponse = self.coordinator.data[DATA_DRIVING_ANALYSIS_KEY]
         return float(da.electric_mileage)
 
+    @property
+    def available(self) -> bool:
+        """Sensor availability."""
+        return self.coordinator.data[DATA_DRIVING_ANALYSIS_KEY] is not None
+
     @property
     def extra_state_attributes(self) -> dict[str, Any]:
         """Return default attributes for Nissan leaf entities."""