Catch more invalid cache source storage paths

OC\Files\Cache\Cache::get can return string|false|null, not just string|false.

- nextcloud/server#26270 added handling of false, but null is needed too.
- Well, or we change the default $resullt to false, but I'm not sure if that has other ramifications and the real need here is to simply catch situations where the cache source storage path is not valid for whatever reason

Related: nextcloud/server#19009

Signed-off-by: Josh Richards <josh.t.richards@gmail.com>

lib/private/Files/Cache/Cache.php

@@ -663,7 +663,7 @@ class Cache implements ICache {
 			$targetPath = $this->normalize($targetPath);
 
 			$sourceData = $sourceCache->get($sourcePath);
-			if ($sourceData === false) {
+			if ($sourceData === false || $sourceData === null) {
 				throw new \Exception('Invalid source storage path: ' . $sourcePath);
 			}