core/cgroup: assigning empty string to Delegate= resets list of controllers (#7336)

core/cgroup: assigning empty string to Delegate= resets list of controllers (#7336)

Before this, assigning empty string to Delegate= makes no change to the
controller list. This is inconsistent to the other options that take list
of strings. After this, when empty string is assigned to Delegate=, the
list of controllers is reset. Such behavior is consistent to other options
and useful for drop-in configs.

Closes #7334.