Close array key gaps in returned arrays

In a lot of places we take the first mount point, first node, etc.

The issue is, that the returned results are handled by array_filter before.
https://github.com/nextcloud/server/blob/b29baababdbc56b1633eca2d8357dd6b87268050/lib/private/Files/Config/UserMountCache.php#L297
https://github.com/nextcloud/server/blob/6fbe991afbb5df36c2fef5cac8c9681f91aed311/lib/private/Files/Node/Folder.php#L326

The problem with that is, that [0] may be undefined. There is currently a bug open in the retention repo: https://github.com/nextcloud/files_retention/pull/38
While this can be easily fixed on the consumer side: https://github.com/nextcloud/files_retention/pull/39
I think we should array_values all the results, to make sure the array keys are lined. We have a lot of consumers of these APIs and all I found did [0] instead of array_shift or something similar. So @rullzer and I think fixing the indexes here is the better idea.

@icewind1991 any other idea?

Start with UserMountCache.php around line 297 and Folder.php around line 326, then inspect the first-mount-point and first-node consumers in RetentionJob.php around lines 163 and 172. Trace the array_filter results and identify the returned APIs that rely on key 0. Done means affected returned arrays have sequential keys so existing consumers cannot encounter an undefined [0].