AsusWRT fix keyerror when firmver is missing from info (#51499)

Co-authored-by: Franck Nijhof <git@frenck.dev>
Co-authored-by: Paulus Schoutsen <paulus@home-assistant.io>

homeassistant/components/asuswrt/router.py

@@ -228,10 +228,10 @@ class AsusWrtRouter:
 
         # System
         model = await _get_nvram_info(self._api, "MODEL")
-        if model:
+        if model and "model" in model:
             self._model = model["model"]
         firmware = await _get_nvram_info(self._api, "FIRMWARE")
-        if firmware:
+        if firmware and "firmver" in firmware and "buildno" in firmware:
             self._sw_v = f"{firmware['firmver']} (build {firmware['buildno']})"
 
         # Load tracked entities from registry