StandaloneMmPkg: Skip zero sized sections while tweaking page permissions

This patch skips zero sized sections in the StMM SP image e.g. .reloc since
there is no point in attempting to change their permissions.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I26e8e2b46790770122bf920b42a5f78cd25d59e9