[bluetooth.bluez] Fix NullPointerException during discovery (#15347)

Signed-off-by: lsiepel <leosiepel@gmail.com>

bundles/org.openhab.binding.bluetooth.bluez/src/main/java/org/openhab/binding/bluetooth/bluez/internal/BlueZBluetoothDevice.java

@@ -469,7 +469,7 @@ public class BlueZBluetoothDevice extends BaseBluetoothDevice implements BlueZEv
     @Override
     public CompletableFuture<byte[]> readCharacteristic(BluetoothCharacteristic characteristic) {
         BluetoothDevice dev = device;
-        if (dev == null || !dev.isConnected()) {
+        if (dev == null || !Boolean.TRUE.equals(dev.isConnected())) {
             return CompletableFuture
                     .failedFuture(new IllegalStateException("DBusBlueZ device is not set or not connected"));
         }