Fix test ConfiguredAppLauncher-LauncherExecutable

Since the setting file [1] is configured with an empty list
of "additionalPathVariables", the associated string list contain an
empty value (See [2]) and trigger the following assert:

ASSERT: "!arrayName.isEmpty()" in file /home/jchris/Projects/CTKAppLauncher/Base/ctkSettingsHelper.cpp, line 13

This commit fix the problem by ignoring empty value.

[1] /home/jchris/Projects/CTKAppLauncher-Debug/Testing/CMake/App4ConfigureLauncherTest-build/./App4ConfigureLauncherTestLauncherSettings.ini
[2] https://bugreports.qt-project.org/browse/QTBUG-19823