zephyr_module.cmake: restore ability to build without modules

Kconfig.modules used to be an empty file when no modules. PR #14667 /
commit bd7569f2 "cmake: Extracted Zephyr module processing into
python script" unintentionally changed that to no Kconfig.modules file
at all when no west and no modules. kconfig.py doesn't like that and
crashes. Restore the empty file.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>