api: Don't return an error when no cgroups are enabled

api: Don't return an error when no cgroups are enabled

Don't return an error, ECGROUPNOTEXIST (50002), from cgroup_get_cgroup()
when no controllers are enabled in a cgroup v2 cgroup.

Simple reproducer:
	sudo mkdir -p /sys/fs/cgroup/foo/bar

	#!/usr/bin/env python3
	from libcgroup import Cgroup, Version

	cg = Cgroup('foo/bar', Version.CGROUP_V2)
	cg.get()
	print(cg)

Prior to this change, the above python code returned ECGROUPNOTEXIST in
the cg.get() line.  Now it successfully completes, but no controllers
are populated.

Reported-by: Justin Israel <justinisrael@gmail.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>