Guard for empty-cache save

Author: bouwew

plugwise_usb/network/cache.py

@@ -31,6 +31,9 @@ async def save_cache(self) -> None:
             for mac, node_type in self._nodetypes.items()
             if node_type.value > 1
         }
+        if not cache_data_to_save:
+            return
+
         _LOGGER.debug("Save NodeTypes for %s Nodes", len(cache_data_to_save))
         await self.write_cache(
             cache_data_to_save, rewrite=True