Fix undefined index and consequential damages in versions code

If the user has no space and there are no versions, there won't be an
`all` index in the versions entry. Hence this triggers a warning and
becomes `null`, afterwards `count`, `foreach` and friends will happily
throw even more warnings and errors because they don't want to play with
`null`. Thus adding a fallback to an empty array.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

apps/files_versions/lib/Storage.php

@@ -776,7 +776,7 @@ class Storage {
 			// if still not enough free space we rearrange the versions from all files
 			if ($availableSpace <= 0) {
 				$result = Storage::getAllVersions($uid);
-				$allVersions = $result['all'];
+				$allVersions = $result['all'] ?? [];
 
 				foreach ($result['by_file'] as $versions) {
 					list($toDeleteNew, $size) = self::getExpireList($time, $versions, $availableSpace <= 0);