Fix falsy values not being returned in post fields (vercel#29011)

I imported a bunch of old markdown posts to the blog-starter example but some of the post metadata was not returned by the API in the example code.

For example, having `published: false` in post metadata was not returned in the item fields.

The problem was in check:
```
if (data[field]) {
  items[field] = data[field]
}
```
This rejects all falsy values in addition to fields that are not set. Checking only for `undefined` should fix this issue.

I didn't find existing integration tests for the blog example nor an issue describing this problem.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

examples/blog-starter-typescript/lib/api.ts

@@ -29,7 +29,7 @@ export function getPostBySlug(slug: string, fields: string[] = []) {
       items[field] = content
     }
 
-    if (data[field]) {
+    if (typeof data[field] !== 'undefined') {
       items[field] = data[field]
     }
   })

examples/blog-starter/lib/api.js

@@ -25,7 +25,7 @@ export function getPostBySlug(slug, fields = []) {
       items[field] = content
     }
 
-    if (data[field]) {
+    if (typeof data[field] !== 'undefined') {
       items[field] = data[field]
     }
   })

examples/blog-with-comment/lib/getPost.js

@@ -25,7 +25,7 @@ export function getPostBySlug(slug, fields = []) {
       items[field] = content
     }
 
-    if (data[field]) {
+    if (typeof data[field] !== 'undefined') {
       items[field] = data[field]
     }
   })