Merge pull request #16 from education/fix-potential-null-issue

add `?` to potential null string

src/main.js

@@ -64,7 +64,7 @@ function run() {
     }
 
     startTime = new Date()
-    output = execSync(command, {timeout, env, stdio: 'inherit'}).toString()
+    output = execSync(command, {timeout, env, stdio: 'inherit'})?.toString()
     endTime = new Date()
 
     result = generateResult('pass', testName, command, output, endTime - startTime, maxScore)