Fix null jsonBody serialization in HttpResponse

[Copilot]

When creating an HTTP response with jsonBody set to null, the value should be serialized as "null" (like JSON.stringify(null)), but it was being treated as undefined and no body or header was being set.

Problem

The issue was in the HttpResponse constructor where it used isDefined(init.jsonBody) which returns false for null values, causing the response to fall back to using init.body instead of properly serializing the null value as JSON.

// Before: This would not serialize null properly
const response = new HttpResponse({ jsonBody: null });
// Result: Empty body with no content-type header

// After: This now works correctly  
const response = new HttpResponse({ jsonBody: null });
// Result: Body contains "null" with application/json content-type

Solution

Changed the condition in HttpResponse constructor from isDefined(init.jsonBody) to init.jsonBody !== undefined to allow null values to be properly serialized as JSON.

Changes Made

• Modified src/http/HttpResponse.ts to check init.jsonBody !== undefined instead of isDefined(init.jsonBody)
• Removed unused isDefined import
• Added comprehensive test cases for JSON primitive types:
  • null serializes to "null" with application/json content-type
  • false serializes to "false" with application/json content-type
  • 0 serializes to "0" with application/json content-type

The fix ensures that all falsy but valid JSON values (null, false, 0) are treated as valid JSON values that should be serialized, consistent with JavaScript's JSON.stringify() behavior.

Fixes #325.

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