fix: allow optional output property in user rollup config

[danielroe]

🔗 Linked issue

❓ 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

This allows users to pass a rollupConfig that doesn't include an output property

📝 Checklist

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

[codecov]

Codecov Report

Merging #751 (900604c) into main (b098cb1) will not change coverage.
The diff coverage is 100.00%.

@@ Coverage Diff @@## main #751 +/- ##
=======================================
Coverage 72.35% 72.35% =======================================
Files 57 57 Lines 5123 5123 Branches 553 553 =======================================
Hits 3707 3707 Misses 1406 1406 Partials 10 10 

Impacted Files	Coverage Δ
src/types/nitro.ts	100.00% <100.00%> (ø)

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