fix: sanitize collection labels to inherit defaults when only a partial config is provided (#13944)

When only a partial `labels` config is defined on a collection, the
collection defaults do not apply as expected. This leads to undefined
`singular` or `plural` properties that render as either empty or
untranslated strings on the front-end.

For example:

```ts
import type { CollectionConfig } from 'payload'

export MyCollection: CollectionConfig = {
  // ... 
  labels: {
    plural: 'Pages', // Notice that `singular` is excluded here
  },
}
```

This renders empty or untranslated strings throughout the admin panel,
here are a couple examples:

<img width="326" height="211" alt="Screenshot 2025-09-26 at 10 27 40 AM"
src="https://github.com/user-attachments/assets/3872c4dd-0dac-4c1c-b417-61ddd042bbb8"
/>

<img width="330" height="267" alt="Screenshot 2025-09-26 at 10 27 51 AM"
src="https://github.com/user-attachments/assets/78772405-b5f3-45fa-9bf0-bc078f1ba976"
/>

---
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used, see below:
  - https://app.asana.com/0/0/1211478736160147

test/config/config.ts

@@ -14,6 +14,10 @@ export default buildConfigWithDefaults({
   collections: [
     {
       slug: 'pages',
+      labels: {
+        // Purposefully exclude `singular` to test default inheritance
+        plural: 'Pages',
+      },
       access: {
         create: () => true,
         delete: () => true,

test/config/int.spec.ts

@@ -126,6 +126,11 @@ describe('Config', () => {
         description: 'The blockOne of this page',
       })
     })
+
+    it('properly merges collection.labels with defaults', () => {
+      const [collection] = payload.config.collections
+      expect(collection?.labels).toEqual({ plural: 'Pages', singular: 'Page' })
+    })
   })
 
   describe('global config', () => {