Null ID for absent pucks

src/main/java/ch/psi/mxsc/Puck.java

@@ -338,6 +338,9 @@ public class Puck extends DeviceBase {
 
     public String getId() {
         //return "XXX000" + index;
+        if (getDetection()!=Detection.Present){
+            return null;
+        }
         return id;
     }