Hi Matthias,
I see the problem. I could suggest two options:
1. In the dropdown input control, delete the value. Then press submit. Image may be NSFW.
Clik here to view.
This works fine, only we don't get the empty value in the list.
2. To have the empty value in the list, like this:
Image may be NSFW.
Clik here to view.
Add to the entries of the list:
Image may be NSFW.
Clik here to view.
Then edit the mapping to the service to convert * into an empty string:
Image may be NSFW.
Clik here to view.
Thanks,
Udi