fix: stub helper utility with no-ops in test mode

[janhoogeveen]

🔗 Linked issue

Resolves #228

❓ 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)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The helper method is initialized as an empty object as a fallback when the helper does not exist on the window object. Later in the code, it's expected that the helper has working methods. This seems to work fine when running a Nuxt app, but not in Vitest.

In this fix I add no-ops and optional chaining to prevent x is not defined on y errors.

[netlify]

✅ Deploy Preview for nuxt-color-mode canceled.

Name	Link
🔨 Latest commit	00e1dc8
🔍 Latest deploy log	https://app.netlify.com/projects/nuxt-color-mode/deploys/69135a01c567b800085c4184

[danielroe]

given that this is only relevant for testing, how about we guard this with if (import.meta.test)?

that way we don't add to the production bundle