Only check the presence of files or includes to allow conditional run…

…nning without logging a warning.

src/sync.js

@@ -61,9 +61,9 @@ async function sync(cfg, opt) {
   // Make include alias includes for backward compat, for now.
   cfg.includes = cfg.includes.concat(cfg.include);
 
-  // If there's no files or includes, it's not really an error but there's
-  // nothing to do.
-  if (!cfg.files.length && !cfg.includes.length) {
+  // If no files or includes are specified, we warn. You can still specify
+  // empty arrays, which explicitly means don't run on anything.
+  if (!("files" in cfg) && !("includes" in cfg)) {
     opt.events.emit(
       "warn",
       'You have not provided any "files" or "includes". For more information see https://github.com/treshugart/conartist#install for ways you can configure conartist.'