MdePkg: Minor update to the Data parameter for PEI GetVariable()

MdePkg: Minor update to the Data parameter for PEI GetVariable()

PI 1.5 spec has minor update to the Data parameter for PEI
GetVariable(), and no real functional behavior change.

The modifier of Data parameter:
OUT VOID *Data
->
OUT VOID *Data OPTIONAL

The comments of Data parameter:
Data    Points to the buffer which will hold the returned variable value.
->
Data    Points to the buffer which will hold the returned variable value.
        May be NULL with a zero DataSize in order to determine the size of the buffer needed.

The patch is to follow the spec to update the definition.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>