Fixed evaluation of application file path

Empty arguments list could lead to crash, due to access of first element.
Besides, empty application file path will be cached now universally.

Change-Id: Ibe1a668da364d87d8431567dfc999cb394686081
Reviewed-by: Sérgio Martins <sergio.martins.qnx@kdab.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>