wrapper.c: Add support for empty values in cgroup_add_value_string()

wrapper.c: Add support for empty values in cgroup_add_value_string()

cgroup_add_value_string() is the fundamental building block
for adding a name/value pair to a cgroup_controller struct.
Add support for a NULL value field.

Currently it only supports adding a valid value, but in a
subsequent commit, cgget will utilize this function to build
up a hierarchy of cgroups, controllers, and setting names.
The values are NULL at the time the struct are populated and
will be read from sysfs at a later time.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>