Add available property to Ambient PWS (#22092)

* Add available property to Ambient PWS

* Linting

* remove blank line

homeassistant/components/ambient_station/__init__.py

@@ -411,6 +411,13 @@ def __init__(
         self._state = None
         self._station_name = station_name
 
+    @property
+    def available(self):
+        """Return True if entity is available."""
+        return bool(
+            self._ambient.stations[self._mac_address][ATTR_LAST_DATA].get(
+                self._sensor_type))
+
     @property
     def device_info(self):
         """Return device registry information for this entity."""