Getting empty variables to return as null instead of " "

Hello, I’ve been trying to upload images to my API and I want to allow some images to be empty. (The user doesn’t have to use all the image spaces) My API expects empty images to return as the word null but appgyver returns it as " " when it’s empty.(I think that’s the problem I’m a complete beginner so I’m not sure. How can I get it to instead return as the word null(no quotations when empty). Any help would be appreciated, thanks in advance.