Visual Studio generates an empty .css file when I compile a .less file. I have this problem in both Visual Studio 2015 and 2017. The file is being compiled because I just wrote something and I can see that it removes the text and overrides it with the empty content.
There are also no error during the compilation. I have Web Essentials and Web Compiler installed (the latest versions because it's updates automatically). I've checked for validation on external online less validators with no errors.
I also tried creating a new .less file and compile it, I get the same empty generated css file.
According to https://github.com/madskristensen/WebCompiler/issues/299 you have to build/rebuild explicitly or right-click on the less file and build. It doesn't happen automatically on save any more.