[homekit] handle NULL/UNDEF for temperature characteristics (#12172)

recent regression from handling QuantityType

Signed-off-by: Cody Cutrer <cody@cutrer.us>

bundles/org.openhab.io.homekit/src/main/java/org/openhab/io/homekit/internal/accessories/HomekitCharacteristicFactory.java

@@ -262,7 +262,7 @@ public class HomekitCharacteristicFactory {
     }
 
     public static @Nullable Double stateAsTemperature(@Nullable State state) {
-        if (state == null) {
+        if (state == null || state instanceof UnDefType) {
             return null;
         }