cgroup: Allow empty assignments of Memory{Low,Min}=

cgroup: Allow empty assignments of Memory{Low,Min}=

Currently, an empty assignment of Memory{Low,Min}= directives would be
interpretted as setting it to global default, i.e. zero. However, if we
set a runtime protection value on a unit that inherits parent's
DefaultMemory{Low,Min}=, it is not possible to revert it back to the
state where the DefaultMemory{Low,Min}= is propagated from parent
slice(s).

This patch changes the semantics of the empty assignments to explicitly
nullify any value set by the user previously. Since DBus API uses
uint64_t where 0 is a valid configuration, the patch modifies DBus API
by exploiting the variant type of property value to pass the NULL value.