MdeModulePkg: Change OPTIONAL keyword usage style

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3760

Update all use of ', OPTIONAL' to ' OPTIONAL,' for function params.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

MdeModulePkg/Library/DebugAgentLibNull/DebugAgentLibNull.c

@@ -34,7 +34,7 @@ VOID
 EFIAPI
 InitializeDebugAgent (
   IN UINT32                InitFlag,
-  IN VOID                  *Context, OPTIONAL
+  IN VOID                  *Context  OPTIONAL,
   IN DEBUG_AGENT_CONTINUE  Function  OPTIONAL
   )
 {
@@ -63,4 +63,3 @@ SaveAndSetDebugTimerInterrupt (
 {
   return FALSE;
 }
-