config: fix some settings incorrectly marked as CFG_DEFAULT_COMMENTED instead of...

config: fix some settings incorrectly marked as CFG_DEFAULT_COMMENTED instead of CFG_DEFAULT_UNDEFINED and causing segfault

This patch fixes segfault which was caused by incorrectly marking some
settings CFG_DEFAULT_COMMENTED instead of CFG_DEFAULT_UNDEFINED - the
ones which have NULL default value, hence they're really undefined.
A regression caused by a98ceceb1d101f0b19a6115f406e23f13feab5ae.

For example:

$ lvmconfig log/file
file="/a"

Before this patch:

$ lvmconfig --type diff
Segmentation fault (core dumped)

With this patch applied:

$ lvmconfig --type diff
log {
	file="/a"
}

The same applies for these settings:

  log/activate_file
  global/library_dir
  global/system_id_file
  <disk_area>/disk_area_id

There were also other settings with NULL default value and marked as
CFG_DEFAULT_COMMENTED instead of CFG_DEFAULT_UNDEFINED, but they were
cfg_array config settings where the NULL value was not causing segfault
(NULL == empty array).