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

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

bundles/org.openhab.binding.lcn/src/main/java/org/openhab/binding/lcn/internal/connection/ConnectionStateMachine.java

@@ -75,7 +75,8 @@ public class ConnectionStateMachine extends AbstractStateMachine<ConnectionState
     public void handleConnectionFailed(@Nullable Throwable e) {
         if (!(state instanceof ConnectionStateShutdown)) {
             if (e != null) {
-                connection.getCallback().onOffline(e.getMessage());
+                String message = e.getMessage();
+                connection.getCallback().onOffline(message != null ? message : "");
             } else {
                 connection.getCallback().onOffline("");
             }