docs(parallel-routes): update 11-parallel-routes.mdx example for modal closing (#72210)

docs(parallel-routes): update 11-parallel-routes.mdx example for modal
closing

- Updated `11-parallel-routes.mdx` to replace `'...'` with `null` in the
`page.tsx` return statement inside the slot for closing the modal when
navigating using `Link`.
- Also applied the same change in the `default.tsx` file to ensure
consistent behavior in the example.


![image](https://github.com/user-attachments/assets/e0c4b2cf-01e1-4cda-8488-11d12a75d32a)

![image](https://github.com/user-attachments/assets/fca6d051-8e43-4022-986c-60557d5da5e6)

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Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>

Author: arfa123

docs/01-app/02-building-your-application/01-routing/11-parallel-routes.mdx

@@ -264,13 +264,13 @@ Then, inside the `@auth` slot, add [`default.js`](/docs/app/api-reference/file-c
 
 ```tsx filename="app/@auth/default.tsx" switcher
 export default function Default() {
-  return '...'
+  return null
 }
 ```
 
 ```jsx filename="app/@auth/default.js" switcher
 export default function Default() {
-  return '...'
+  return null
 }
 ```
 
@@ -433,27 +433,27 @@ export function Modal({ children }) {
 
 ```tsx filename="app/@auth/page.tsx" switcher
 export default function Page() {
-  return '...'
+  return null
 }
 ```
 
 ```jsx filename="app/@auth/page.js" switcher
 export default function Page() {
-  return '...'
+  return null
 }
 ```
 
 Or if navigating to any other page (such as `/foo`, `/foo/bar`, etc), you can use a catch-all slot:
 
 ```tsx filename="app/@auth/[...catchAll]/page.tsx" switcher
 export default function CatchAll() {
-  return '...'
+  return null
 }
 ```
 
 ```jsx filename="app/@auth/[...catchAll]/page.js" switcher
 export default function CatchAll() {
-  return '...'
+  return null
 }
 ```