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.

src/shared/bus-unit-util.c

@@ -489,11 +489,24 @@ static int bus_append_cgroup_property(sd_bus_message *m, const char *field, cons
                               "MemoryLimit",
                               "TasksMax")) {
 
-                if (isempty(eq) || streq(eq, "infinity")) {
+                if (streq(eq, "infinity")) {
                         r = sd_bus_message_append(m, "(sv)", field, "t", CGROUP_LIMIT_MAX);
                         if (r < 0)
                                 return bus_log_create_error(r);
                         return 1;
+                } else if (isempty(eq)) {
+                        uint64_t empty_value = STR_IN_SET(field,
+                                                          "DefaultMemoryLow",
+                                                          "DefaultMemoryMin",
+                                                          "MemoryLow",
+                                                          "MemoryMin") ?
+                                               CGROUP_LIMIT_MIN :
+                                               CGROUP_LIMIT_MAX;
+
+                        r = sd_bus_message_append(m, "(sv)", field, "t", empty_value);
+                        if (r < 0)
+                                return bus_log_create_error(r);
+                        return 1;
                 }
 
                 r = parse_permille(eq);