FIX EZP-22472: Incorrect handling of dfs cache error

JIRA: https://jira.ez.no/browse/EZP-22472

After the fix for https://jira.ez.no/browse/EZP-21547 (see https://github.com/ezsystems/ezpublish-legacy/commit/57171cd9), there is the possibility that `eZClusterFileHandler::processCache()` will result in a `eZClusterFileFailure` instance.

The next lines are using the result as an array, which (in case of error) will not be valid PHP.
This fix simply adds a check before using `$result` as an `array`.