Fix: protect search-data.json file from front matter default for layout (#1468)

Fixes #1466. Prior art: #1447. Otherwise self-explanatory.

To test:

1. First, clone [template repository](https://github.com/just-the-docs/just-the-docs-template/tree/main). Observe that search works.
2. Next, add a default layout to all files
```yml
defaults:
  - scope:
      path: ""
    values:
      layout: "default"
```
3. Observe that search no longer works.
4. Apply this patch
5. Observe that search works again!

assets/js/zzzz-search-data.json

@@ -1,4 +1,5 @@
 ---
+layout: null
 permalink: /assets/js/search-data.json
 ---
 {