Most photographs are pictures of something. There's an object, or a scene, a juxtapostition of shapes, something photographically promising which was recorded in the frame. Snapshots are almost exclusively this. Portraits are by definition this. And so forth.
Some photographs, a few, are photographs of nothing. That is, they're a photograph of whatever was in front of the lens at the time, but the point is that, whatever that was, it was photographically "nothing." Examples might be a texture, a river-smoothed stone indistinguishable from the rest, or a dimly lit green pepper. There might be a subject, but it's sufficiently crappy and uninteresting that it's "nothing" as a subject.
Obviously this is something of a subjective call, and what we're really talking about is a spectrum from "really interesting subject" to "really uninteresting subject" or something similar. Regardless, let us press on, and recognize that for most of us there are photographs we would characterize as "photographs of nothing" although we might disagree on which photographs those are.
Within that realm of photographs of nothing, I distinguish two kinds. This distinction might be completely artificial, or so subtle as to be a stupid bit of trivial parsing. Nonetheless, it's a distinction important enough to me that I tend to like the one kind, and tend to dislike the other kind.
The first kind of photograph of nothing attempts to reveal the nothing as, in fact, something. The very best example of this is probably Weston's "Pepper No. 30" which does, in fact, reveal a humble green pepper as something. Bad examples of this form litter the art galleries of the western world. Endless stupid pictures of abstract crap that the viewer is supposed to, somehow, come to love.
The second kind of photograph of nothing does not attempt to reveal the nothing as something. It lets the nothing alone, leaves it in the frame to be humble, ugly, and not photogenic at all. The successful examples of this form create, in spite of the humble subject, a fine image. A good example of this is Evans' photograph of the Gudger's mantel, from the book Let Us Now Praise Famous Men. This is a photograph of a rather unattractive mantel in the home of an indescribably poor family. The mantel has on it many of the family's small possessions, objects which are almost literally garbage. Despite the unpromising subjects, Evans made an iconic and powerful photograph. A good photograph.
Photographs of nothing are absurdly hard to make well. When attempting the first kind, one is likely to fall short. You might find a small audience that gets it, that sees the beauty of mud blobs revealed by your photographs of mud blobs (why are there always a bunch of these photographs instead of one good one?) It's very hard to make images like this that are powerful and evocative for more than a few people. When attempting the second kind, it's even worse, since there is little or nothing to love inside the frame, one must love the frame as a whole. Thus, the photographer has no room for error, the whole must work at once, and must overcome the basic nothingness of the stuff in-frame, or it fails completely.
On the positive side, if you can do it, perhaps you can do anything.