cmake: modules: Remove duplicates only from existing ZEPHYR_MODULE_NAMES

Remove duplicates from ZEPHYR_MODULE_NAMES only if that list
exists. That avoids cmake error "list sub-command REMOVE_DUPLICATES
requires list to be present" if ZEPHYR_MODULE_NAMES is empty.

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>