I'm afraid that the availability of the serial number in that property depends if the HW vendor made that information available according the specs that VMware expects it to be.
With IBM HW that is unfortunately not always the case;
- most x3650 M2 have the serial number in that property,
- some older x3650 M3 do not have the serial number in that property
- with the x3650 M3 I never saw the serial number in that property
The availability varies in the same way with other models from IBM
