fix: return undefined instead of null for non existing keys

[pi0]

🔗 Linked issue

Resolves #205

❓ 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 rework of #206

By changes to this PR, storage returns undefined instead of null when a key does not exist.

On the downside, this change makes unstorage less consistent with local storage and Cloudflare KV when getItem returns null when the key does not exist.

On the benefits, more thinking this is less a breaking change for users but actually a fix since we were already listing null as a valid Storage value (which does exist!). And we reduce use of evil null. (sindresorhus/meta#7)

📝 Checklist

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

[nuxt-studio]

✅ Live Preview ready!

Name	Edit	Preview	Latest Commit
unstorage	Edit on Studio ↗︎	View Live Preview	c9ca0ba

[codecov]

Codecov Report

Merging #260 (c9ca0ba) into main (cb8577b) will increase coverage by 0.06%.
The diff coverage is 65.00%.

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
+ Coverage   76.27%   76.33%   +0.06%     
==========================================
  Files          25       25              
  Lines        3030     3064      +34     
  Branches      441      451      +10     
==========================================
+ Hits         2311     2339      +28     
- Misses        718      724       +6     
  Partials        1        1              

Impacted Files	Coverage Δ
src/drivers/azure-app-configuration.ts	40.68% <0.00%> (ø)
src/drivers/azure-cosmos.ts	44.05% <0.00%> (ø)
src/drivers/azure-key-vault.ts	35.76% <0.00%> (-1.08%)	⬇️
src/drivers/azure-storage-blob.ts	33.33% <0.00%> (-0.38%)	⬇️
src/drivers/azure-storage-table.ts	43.93% <80.00%> (ø)
src/drivers/cloudflare-kv-binding.ts	83.90% <100.00%> (+3.08%)	⬆️
src/drivers/cloudflare-kv-http.ts	84.47% <100.00%> (+1.06%)	⬆️
src/drivers/github.ts	89.41% <100.00%> (ø)
src/drivers/localstorage.ts	80.00% <100.00%> (ø)
src/drivers/lru-cache.ts	91.42% <100.00%> (ø)
... and 7 more

[pi0]

PR stalled. Let's track via #205