Docs: Improve recipe for empty glob array (closes #2122)

Author: contra

Diff for: docs/recipes/running-task-steps-per-folder.md

@@ -36,9 +36,9 @@ function getFolders(dir) {
       });
 }
 
-gulp.task('scripts', function() {
+gulp.task('scripts', function(done) {
    var folders = getFolders(scriptsPath);
-
+   if (folder.length === 0) return done(); // nothing to do!
    var tasks = folders.map(function(folder) {
       return gulp.src(path.join(scriptsPath, folder, '/**/*.js'))
         // concat into foldername.js