SmartString -> CString doesn't error in match cases but causes incorrect C code

[alex-s168]

I wish I could create a PR for once without first having to file issues lol

This doesn't error:

let .file-extension(path: CString): CString = (
   let dot = path.find-last(46_u8);
   match dot {
      Some{content=content} => path.substring(content + 1);
      None{} => "";
   }
);

but generates incorrect C code because "" is a SmartString, and the function should return a CString

Code to reproduce here