[deconz] Set "ontime" option only if thing has got an ONTIME channel (#10829)

* Set "ontime" option only if thing has got an ONTIME channel

Fixes #10774

Signed-off-by: Martin Herbst <develop@mherbst.de>

bundles/org.openhab.binding.deconz/src/main/java/org/openhab/binding/deconz/internal/handler/LightThingHandler.java

@@ -96,7 +96,8 @@ public class LightThingHandler extends DeconzBaseThingHandler {
      */
     private LightState lightStateCache = new LightState();
     private LightState lastCommand = new LightState();
-    private int onTime = 0; // in 0.1s
+    @Nullable
+    private Integer onTime = null; // in 0.1s
     private String colorMode = "";
 
     // set defaults, we can override them later if we receive better values