Fix backup cache handling when JSON decode returns null

[Dominik0101]

The WebUI crashes 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.

[theCalcaholic]

Thanks so much, lgtm

[theCalcaholic]

The pipelines don't seem to be working on a different repo, so I'll merge this into a temporary branch for testing. :)
It's a small enough change, so I don't expect anything to come up

[theCalcaholic]

Continued in #2113