cgroup: Make empty assignments reset to default

When MemoryLow= or MemoryMin= is set, it is interpretted as setting the
values to infinity. This is inconsistent with the default initialization
to 0.
It'd be nice to interpret the empty assignment as fallback to
DefaultMemory* of parent slice, however, current DBus API cannot convey
such a NULL value, so stick to simply interpretting that as hard-wired
default.