[infrastructure] add external null-annotations (#8848)

Signed-off-by: Jan N. Klug <jan.n.klug@rub.de>

bundles/org.openhab.binding.knx/src/main/java/org/openhab/binding/knx/internal/client/AbstractKNXClient.java

@@ -213,8 +213,9 @@ public abstract class AbstractKNXClient implements NetworkLinkListener, KNXClien
     private void disconnect(@Nullable Exception e) {
         releaseConnection();
         if (e != null) {
+            String message = e.getLocalizedMessage();
             statusUpdateCallback.updateStatus(ThingStatus.OFFLINE, ThingStatusDetail.COMMUNICATION_ERROR,
-                    e.getLocalizedMessage());
+                    message != null ? message : "");
         } else {
             statusUpdateCallback.updateStatus(ThingStatus.OFFLINE);
         }