Do not try tp load invalid scripts.

Summary:
It can happen, that if the server supports KEP:14, the USER file is the only
file that exists and that it is empty. In case we are at the end of the list
of avaialbe scripts, we should not try to load a next element from the list.

There is no vacation script in KEP:14 special files

Reviewers: mollekopf

Maniphest Tasks: T1183

Differential Revision: https://git.kolab.org/D142