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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