fix: there's a command response format change in nightwatch v2

nightwatchjs/nightwatch#2821

Author: haoqunjiang

packages/@vue/cli-plugin-e2e-nightwatch/generator/template/tests/e2e/custom-commands/openHomepage.js

@@ -18,6 +18,6 @@ module.exports = {
     this.waitForElementVisible('#app')
 
     const result = await this.elements('css selector', '#app ul')
-    this.assert.strictEqual(result.value.length, 3)
+    this.assert.strictEqual(result.length, 3)
   }
 }