fix(graphql): nextPage and prevPage are non nullable even though they can be null sometimes (#12201)

This PR introduced https://github.com/payloadcms/payload/pull/11952
improvement for graphql schema with making fields of the `Paginated<T>`
interface non-nullable.

However, there are a few special ones - `nextPage` and `prevPage`. They
can be `null` when:
The result returned 0 docs.
The result returned `x` docs, but in the DB we don't have `x+1` doc.
Thus, `nextPage` will be `null`. The result will have `nextPage: null`.
Finally, when we query 1st page, `prevPage` is `null` as well.

<img width="873" alt="image"
src="https://github.com/user-attachments/assets/04d04b13-ac26-4fc1-b421-b5f86efc9b65"
/>

test/graphql/int.spec.ts

@@ -104,5 +104,50 @@ describe('graphql', () => {
 
       expect(res.hyphenated_name).toStrictEqual('example-hyphenated-name')
     })
+
+    it('should not error because of non nullable fields', async () => {
+      await payload.delete({ collection: 'posts', where: {} })
+
+      // this is an array if any errors
+      const res_1 = await restClient
+        .GRAPHQL_POST({
+          body: JSON.stringify({
+            query: `
+query {
+  Posts {
+    docs {
+      title
+    }
+    prevPage
+  }
+}
+        `,
+          }),
+        })
+        .then((res) => res.json())
+      expect(res_1.errors).toBeFalsy()
+
+      await payload.create({
+        collection: 'posts',
+        data: { title: 'any-title' },
+      })
+
+      const res_2 = await restClient
+        .GRAPHQL_POST({
+          body: JSON.stringify({
+            query: `
+query {
+  Posts(limit: 1) {
+    docs {
+      title
+    }
+  }
+}
+        `,
+          }),
+        })
+        .then((res) => res.json())
+      expect(res_2.errors).toBeFalsy()
+    })
   })
 })