cmake: fix zephyr_get_system_include_directories_for_lang() function

This commit fixes the zephyr_get_system_include_directories_for_lang()
function in the event that no system include directories are specified
for Zephyr interface.

This fixes an issue where the value returned by this function is
`-isystem` with no directory.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>