fix: allow use with script tag

[wattanx]

🔗 Linked issue

Fixes: #952

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

When using script setup in Vue 2, there may be cases where the script tag is used in conjunction.
If export default is declared in script, then

<script setup>
definePageMeta({
  middleware: ['test']
})
</script>

<script>
export default {
  name: 'Test'
}
</script>

transformed(vite):

const __default__ = {
  name: "Test"
}

export default /* @__PURE__ */ _defineComponent({
  ...__default__,
  setup(__props) {
    definePageMeta({
      middleware: ["test"]
    });
    return { __sfc: true };
  }
});

The content of export default is SpreadElement and there is no key.
So I fixed it to check if there is a key or not.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.