Set Synology DSM update entity to unavailable in case no data from api gathered (#79508)

homeassistant/components/synology_dsm/update.py

@@ -52,6 +52,11 @@ class SynoDSMUpdateEntity(SynologyDSMBaseEntity, UpdateEntity):
     entity_description: SynologyDSMUpdateEntityEntityDescription
     _attr_title = "Synology DSM"
 
+    @property
+    def available(self) -> bool:
+        """Return True if entity is available."""
+        return bool(self._api.upgrade)
+
     @property
     def installed_version(self) -> str | None:
         """Version installed and in use."""