How can I express "I can't find any papers written about this"?

(disclaimer - I come from computer science, and the little I know about conventions in philosophy is from hearsay)

I'd like to be able to say "I looked around on Google scholar, etc, and I couldn't find anything about this topic"

The usual expression for this kind of thing in my field is "to the best of our knowledge, this topic/idea has so far not been considered in literature". It tells the reader that you think that nobody has published this idea yet, but clearly the absence of existing literature is something very difficult to be sure about. Hence the rather defensive formulation.

In general, is there a technique to demonstrate the absence of literature about a certain topic?

Nope. How would you do that? It's a bit like proving a hypothesis - you can only find counterexamples (papers that do cover the topic), but you cannot prove that in some weird journal nobody reads somebody has expressed your thoughts already.

What kind of evidence is appropriate in this situation? Would providing something like the number of results of a search on Google scholar be worthwhile?

The accepted convention in my field is to explain the methodology of how you searched for literature (you had a defined methodology, right?), and then explain / list / discuss what you found. Be specific - which keywords did you use and why? Why Google Scholar and not something else? Can you be sure that all papers that you would expect to be relevant are indexed by Scholar (some of those should be pretty old, long before the Internet was a thing)? Did you also check your library for dead-tree literature? Number of results and or similar metrics seem pretty much useless to me and would likely not improve your paper in any way.

The most relevant works we found are (cite the publication where the water first have been mentioned when your publication is about the invention of the submarine) however they (and say why actually not too relevant).

This will not protect from the deserved criticism if the relevant publications do exist.

This is not a direct answer to your actual question. Such has been well provided above. Rather, I humbly suggest, as someone who majored in philosophy and has been burned repeatedly by the notion of originality, that when you are tempted to think "it's possible that the midterm paper that you're [writing] is bumping up against an original area of research," refresh yourself with a jaunt to your school's library. There you might find such gems as Benoit B. Mandelbrot's The Fractal Geometry of Nature (San Francisco: W.H. Freeman, 1982) and The Philosophy of Leibniz: Metaphysics and Language by Benson Mates (New York: Oxford University Press, 1989), both of which treat, at least briefly, Leibniz' historical contribution to the development of graph theory. Your library may also have less widely distributed journals and "unpublished" works which are not yet well cataloged on them interwebs.

You might also find that some good scholarly work is done in introductions to collections of original writings. Leroy E Loemker's intro to Philosophical Papers and Letters: A Selection, Gottfried Wilhelm Leibniz (Dordrecht, Holland; Boston: D. Reidel Pub. Co., 1976, 1969) is one such which just so happens to mention your topic. You might also find inspiration in proximal works on such topics as Turing, computational logic, machine theory, and linguistics, to say nothing of the huge body of literature dealing with the interrelationship of analysis situs, graph theory, topology, and calculus.

At the very least, the ability to reference prior works that broach your subject will lend legitimacy to what you might build upon them.

If you're really interested in this subject, though, your best bet is probably to check your syllabus for your professor's office hours, and then drop by for a few minutes to briefly explain your topic, along with your difficulty finding references, and ask her if she has any suggestions as to where you might turn. But I wouldn't mention google.