Prevent update entity becoming unavailable on device disconnect in IronOS (#129840)

* Don't render update entity unavailable when Pinecil device disconnects

* fixes

homeassistant/components/iron_os/update.py

@@ -92,4 +92,7 @@ class IronOSUpdate(IronOSBaseEntity, UpdateEntity):
     @property
     def available(self) -> bool:
         """Return if entity is available."""
-        return super().available and self.firmware_update.last_update_success
+        return (
+            self.installed_version is not None
+            and self.firmware_update.last_update_success
+        )