Fix empty challenge directory error

It occured after moving back from challenge branch to the main.

scripts/build.js

@@ -1,8 +1,16 @@
 const { readdirSync } = require('fs')
+const { resolve } = require('path')
+
+const isDirEmpty = (directory, subdirectory) => {
+  const path = resolve(directory, subdirectory)
+  return readdirSync(path).length === 0
+}
 
 const getDirectories = (source) =>
   readdirSync(source, { withFileTypes: true })
-    .filter((dirent) => dirent.isDirectory())
+    .filter(
+      (dirent) => dirent.isDirectory() && !isDirEmpty(source, dirent.name)
+    )
     .map((dirent) => dirent.name)
 
 const foreachChallenge = (command) => {