fix(http-request): prevent Content-Type header for empty string body

Fix critical bug where empty string body incorrectly triggered
Content-Type header to be set.

Bug Details:
- Changed condition from `if (body !== undefined)` to `if (body)`
- Empty string "" is !== undefined (true) but is falsy
- Sending Content-Type with empty body is semantically incorrect
- Could break APIs that treat empty bodies differently

Affected Functions:
- httpJson(): Line 1011 (now uses truthiness check)
- httpText(): Line 1185 (now uses truthiness check)

Test Coverage:
- Added test: should not set Content-Type when body is empty string (httpJson)
- Added test: should not set Content-Type when body is empty string (httpText)
- Total tests: 69 (up from 67)

Impact: High - Affects POST/PUT/PATCH requests with empty string bodies

Closes critical issue identified in code review.

Author: jdalton

src/http-request.ts

@@ -1008,7 +1008,7 @@ export async function httpJson<T = unknown>(
   }
 
   // Add Content-Type when body is present
-  if (body !== undefined) {
+  if (body) {
     defaultHeaders['Content-Type'] = 'application/json'
   }
 
@@ -1182,7 +1182,7 @@ export async function httpText(
   }
 
   // Add Content-Type when body is present
-  if (body !== undefined) {
+  if (body) {
     defaultHeaders['Content-Type'] = 'text/plain'
   }
 

test/unit/http-request.test.mts

@@ -988,6 +988,37 @@ describe('http-request', () => {
       }
     })
 
+    it('should not set Content-Type when body is empty string', async () => {
+      const testServer = http.createServer((req, res) => {
+        res.writeHead(200, { 'Content-Type': 'application/json' })
+        res.end(
+          JSON.stringify({ contentType: req.headers['content-type'] || null }),
+        )
+      })
+
+      await new Promise<void>(resolve => {
+        testServer.listen(0, () => resolve())
+      })
+
+      const address = testServer.address()
+      const testPort = address && typeof address === 'object' ? address.port : 0
+
+      try {
+        const data = await httpJson<{ contentType: string | null }>(
+          `http://localhost:${testPort}/`,
+          {
+            method: 'POST',
+            body: '',
+          },
+        )
+        expect(data.contentType).toBeNull()
+      } finally {
+        await new Promise<void>(resolve => {
+          testServer.close(() => resolve())
+        })
+      }
+    })
+
     it('should allow overriding default headers', async () => {
       const testServer = http.createServer((req, res) => {
         res.writeHead(200, { 'Content-Type': 'application/json' })
@@ -1169,6 +1200,32 @@ describe('http-request', () => {
       }
     })
 
+    it('should not set Content-Type when body is empty string', async () => {
+      const testServer = http.createServer((req, res) => {
+        res.writeHead(200, { 'Content-Type': 'text/plain' })
+        res.end(req.headers['content-type'] || 'no-content-type')
+      })
+
+      await new Promise<void>(resolve => {
+        testServer.listen(0, () => resolve())
+      })
+
+      const address = testServer.address()
+      const testPort = address && typeof address === 'object' ? address.port : 0
+
+      try {
+        const text = await httpText(`http://localhost:${testPort}/`, {
+          method: 'POST',
+          body: '',
+        })
+        expect(text).toBe('no-content-type')
+      } finally {
+        await new Promise<void>(resolve => {
+          testServer.close(() => resolve())
+        })
+      }
+    })
+
     it('should allow overriding default headers', async () => {
       const testServer = http.createServer((req, res) => {
         res.writeHead(200, { 'Content-Type': 'text/plain' })