NetworkPkg/UefiPxeBcDxe: Allow the NULL configuration for NewStationIP/NewSubnetMask

NetworkPkg/UefiPxeBcDxe: Allow the NULL configuration for NewStationIP/NewSubnetMask

According the UEFI Spec for PxeBc.SetStationIP():
If NewStationIP is NULL, then the current IP address will not be modified.
...
If NewSubnetMask is NULL, then the current subnet mask will not be modified.

Currently, EfiPxeBcSetStationIP() doesn't comply with UEFI Spec. This patch is
to fix the issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>