fix null error in getUnencryptedSize

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

lib/private/Files/Cache/CacheEntry.php

@@ -137,7 +137,7 @@ class CacheEntry implements ICacheEntry {
 		if (isset($this->data['unencrypted_size']) && $this->data['unencrypted_size'] > 0) {
 			return $this->data['unencrypted_size'];
 		} else {
-			return $this->data['size'];
+			return $this->data['size'] ?? 0;
 		}
 	}
 }