MdeModulePkg Variable: 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 implementation.

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@19771 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c

@@ -2822,7 +2822,8 @@ Done:
   @param Attributes                 Attribute value of the variable found.
   @param DataSize                   Size of Data found. If size is less than the
                                     data, this value contains the required size.
-  @param Data                       Data pointer.
+  @param 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.
 
   @return EFI_INVALID_PARAMETER     Invalid parameter.
   @return EFI_SUCCESS               Find the specified variable.
@@ -2837,7 +2838,7 @@ VariableServiceGetVariable (
   IN      EFI_GUID          *VendorGuid,
   OUT     UINT32            *Attributes OPTIONAL,
   IN OUT  UINTN             *DataSize,
-  OUT     VOID              *Data
+  OUT     VOID              *Data OPTIONAL
   )
 {
   EFI_STATUS              Status;