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

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

UEFI 2.6 spec has minor update to the Data parameter for 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	The buffer to return the contents of the variable.
->
Data	The buffer to return the contents of the variable. May be NULL
        with a zero DataSize in order to determine the size 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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19770 6f19259b-4bc3-4df7-8a09-765794883524