Fix backup cache handling when JSON decode returns null

[Dominik0101]

The backups page can crash when the cache file contains invalid or empty JSON.

This happens because json_decode($cache_str, true) may return null.
The previous implementation used or [ ], which does not assign the fallback array as intended due to PHP operator precedence.

This patch ensures that $cache is always initialized as an array:

• If json_decode returns a valid array, it is used
• Otherwise, $cache falls back to an empty array

This prevents runtime errors when accessing the cache.

[Dominik0101]

The issue can be reproduced by opening the backups section in the NextcloudPi admin panel.

If the cache file contains invalid or empty JSON, the page fails with:
"Something went wrong. Try refreshing the page"