Fix "Call to a member function getUID() on null"

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

apps/dav/lib/Files/RootCollection.php

@@ -21,6 +21,7 @@
  */
 namespace OCA\DAV\Files;
 
+use Sabre\DAV\INode;
 use Sabre\DAVACL\AbstractPrincipalCollection;
 use Sabre\HTTP\URLUtil;
 use Sabre\DAV\SimpleCollection;