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.