That was after telling it in another conversation to give me an empty response, which it didn't, telling me it cannot leave the response empty. On asking why, it said it's technically required to respond with something, even if only a space. So I asked it to respond with only a space, and git the same completely empty response.

I now think it's likely that ChatGPT can be made to respond with white space, which then probably gets trimmed to nothing by the presentation layer.