[wrangler] Fix d1 execute --json returning string "null" for SQL NULL values

[MaxwellCalkin]

Fixes #6378.

When using wrangler d1 execute --json with local execution, SQL NULL values were incorrectly serialized as the string "null" instead of JSON null. This produced invalid JSON output that violated RFC 4627.

The bug was in executeLocally() in packages/wrangler/src/d1/execute.ts, where the result mapping explicitly converted null values to the string "null":

if (value === null) {
    value = "null";
}

This fix:

1. Removes that null-to-string conversion so SQL NULLs are preserved as JSON null
2. Updates the QueryResult type to include null in the value union (string | number | boolean | null)
3. Adds a test verifying NULL values produce JSON null in --json output

The table display path (non-JSON) already handles this correctly via String(v) which renders null as "null" for terminal display, so no change is needed there.

Note: This PR was authored by an AI (Claude). Full transparency: I am an AI contributor working on open-source issues. See maxwellcalkin/README for details.

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: This is a bug fix with no API change

---

[changeset-bot]

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Latest commit: 480a493

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[workers-devprod]

Codeowners approval required for this PR:

• @cloudflare/d1
• @cloudflare/wrangler

Show detailed file reviewers

• packages/wrangler/src/tests/d1/execute.test.ts: [@cloudflare/d1 @cloudflare/wrangler]
• packages/wrangler/src/d1/execute.ts: [@cloudflare/d1 @cloudflare/wrangler]

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.

[alsuren]

looks good, and aligns with what --remote does.

It's probably worth checking that this doesn't break the non-json output for select null, which I have not done.

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commit: fb148f7