fix(resolve): ignore null export targets

[kiki-kanri]

Fixes lookupNodeModuleSubpath crashing when a package exports map contains null targets.

null is a valid package exports target for disabling a condition or subpath. Previously _flattenExports recursively processed non-string values, so null targets eventually reached
Object.entries(null) and threw:

TypeError: Cannot convert undefined or null to object

This change treats null export targets as omitted while flattening exports.

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of package exports when exports are null, preventing potential errors in export resolution.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 91c543c0-7b31-4d95-a23c-be5ebb3293e1

📥 Commits

Reviewing files that changed from the base of the PR and between beece9b and 27c4c13.

⛔ Files ignored due to path filters (1)

• test/fixture/package/node_modules/subpaths/package.json is excluded by !**/node_modules/**

📒 Files selected for processing (1)

• src/resolve.ts

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📝 Walkthrough

Walkthrough

Updated the _flattenExports internal helper to accept null exports and return an empty flattened array in that case, improving null-safety without changing behavior for non-null export maps.

Changes

Export flattening robustness

Layer / File(s)	Summary
Null-safe export flattening src/resolve.ts	The _flattenExports function signature now accepts null exports values and immediately returns an empty array when exports is null, while preserving all existing flattening behavior for non-null export maps.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

A rabbit hops through exports with care,
Where null once lurked in the air,
Now a guard clause stands tall and strong,
Making flattening safe all along! 🐰✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix(resolve): ignore null export targets' clearly and specifically summarizes the main change—handling null values in export targets to prevent crashes during module resolution.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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