fix: handle null serialization

[wuiyang]

🔗 Linked issue

Resolves #636

❓ 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

This PR adds condition check during serialization, to avoid serialize null value as object, as typeof null === 'object'.
It fix serialization issue for @nuxtjs/apollo config provided in nuxt.config.ts, where it cannot handle serialization for null value, which is accepted value for client.authType.

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