fix(files): Don't try to use missing cache entries

The `Cache::get()` function can return `ICacheEntry` or `false`.
Added check ensures that `false` return value is not used as an array.
This silences: `Trying to access array offset on value of type bool` errors.

Signed-off-by: Jacek Tomasiak <jacek.tomasiak@gmail.com>

lib/private/Files/Cache/Cache.php

@@ -870,7 +870,7 @@ public function correctFolderSize($path, $data = null, $isBackgroundScan = false
 			}
 			if ($isBackgroundScan) {
 				$parentData = $this->get($parent);
-				if ($parentData['size'] !== -1 && $this->getIncompleteChildrenCount($parentData['fileid']) === 0) {
+				if ($parentData !== false && $parentData['size'] !== -1 && $this->getIncompleteChildrenCount($parentData['fileid']) === 0) {
 					$this->correctFolderSize($parent, $parentData, $isBackgroundScan);
 				}
 			} else {