I still think you're going about this the wrong way.
From the clipboard data you supplied earlier, it looks like there will only be an image when there's an image link. So try searching for
"public.html作为数据": "<img src=\"file:///var/folders/
...or similar. If that's true -- image. If not -- no image.
Otherwise, I think it's confusion about regex101's use of NULL. I think they use that to indicate "the match group is an empty string" so you could try the usual KM regex->variables then test the variables with "If... is empty".