fix(types): account for undefined value for $fetch return type

[DamianGlowala]

🔗 Linked issue

nuxt/nuxt#26411

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 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 is a continuation of #2286 which will fix not inferring undefined from server routes (checked this locally and works great).

The origin of the Exclude wrapper:
https://github.com/unjs/nitro/blame/adb6c5d1c7820feef63ed0851d7850f04d98ebb1/src/types/fetch.ts#L13

Confirmed with @danielroe whether there was any specific reason for excluding Error | void and he's not aware (hence it might be redundant but worth double checking).

📝 Checklist

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

[pi0]

Thanks for the PR dear @DamianGlowala but is there any hope that you please help on providing minimal Nitro reproduction or update the fixtures?

Even though i trust you already well tested this enough it doesn't feel right to merge changes untested like this.

[DamianGlowala]

Sure Pooya! Tried adding a fixture, but MiddlewareOf heavily depends on other types and ended up preparing a reproduction you can have a look at.

Launch locally the following reproduction, inspect type of $fetch within server/routes/test.ts, remove the Exclude wrapper underlined in red, go back to test.ts and hover over the response variable observing how undefined is correctly inferred.