bmcweb: Redfish make MemorySizeInKB optional

This commit makes Memory Size an optional parameter.
On X86 platforms, MemorySizeInKB is coming from the MDR daemon.
For other platforms it is undefined. Still some work to do here,
but this commit fixes the internal error in
/redfish/v1/Systems/{ComputerSystemId}/Memory/{MemoryId} on non-X86
systems.

Resolves: openbmc/bmcweb#19

curl -k -H "X-Auth-Token: $bmc_token" -X GET \
https://${bmc}/redfish/v1/Systems/motherboard/Memory/dimm9
{
  "@odata.context": "/redfish/v1/$metadata#Memory.Memory",
  "@odata.id": "/redfish/v1/Systems/motherboard/Memory/dimm9",
  "@odata.type": "#Memory.v1_2_0.Memory",
  "Id": "dimm9",
  "Name": "DIMM Slot",
  "Status": {
    "Health": "OK",
    "State": "Enabled"
  }
}

Change-Id: Ib2b558ba2299674edab0132a21dc6109e4b81732
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>