OvmfPkg/MemDebugLogLib: Add MemDebugLogInit/Copy stubs to Null instance

The Null instance of MemDebugLogLib only provides MemDebugLogWrite(),
MemDebugLogPages(), and MemDebugLogEnabled(). Any module that
references MemDebugLogInit() or MemDebugLogCopy() cannot link against
the Null instance, forcing the feature to be gated at the build-system
level rather than at runtime.

Add no-op stubs for MemDebugLogInit() and MemDebugLogCopy().

This will be used by the following commit to add memory debug log
support in TDX.

Signed-off-by: Luigi Leonardi <leonardi@redhat.com>

OvmfPkg/Library/MemDebugLogLib/MemDebugLogNull.c

@@ -38,3 +38,23 @@ MemDebugLogEnabled (
 {
   return FALSE;
 }
+
+EFI_STATUS
+EFIAPI
+MemDebugLogInit (
+  IN EFI_PHYSICAL_ADDRESS  MemDebugLogBufAddr,
+  IN UINT32                MemDebugLogBufSize
+  )
+{
+  return EFI_SUCCESS;
+}
+
+EFI_STATUS
+EFIAPI
+MemDebugLogCopy (
+  IN EFI_PHYSICAL_ADDRESS  MemDebugLogBufDestAddr,
+  IN EFI_PHYSICAL_ADDRESS  MemDebugLogBufSrcAddr
+  )
+{
+  return EFI_SUCCESS;
+}