StandaloneMmPkg: Avoid updating attributes on 0 length sections

PE images can have a zero virtual length section for placeholders, unused
relocation, or other artifacts. At best, updated attributes on such sections
is a no-op, but at worst is can cause spurious errors and asserts. Specifically
asserts have been observed in Hafnium when attempting to update attributes
on a zero length section. It is best to just skip such sections entirely.

This commit simply logs and skips these sections explicitly.

Signed-off-by: Chris Fernald <chfernal@microsoft.com>