return the correct value when trying to get a non existing item from cache by id

Signed-off-by: Robin Appelman <robin@icewind.nl>

lib/private/Files/Cache/Cache.php

@@ -150,6 +150,8 @@ class Cache implements ICache {
 				$data = $this->partial[$file];
 			}
 			return $data;
+		} else if (!$data) {
+			return $data;
 		} else {
 			return self::cacheEntryFromData($data, $this->mimetypeLoader);
 		}