pldm: prefer empty() over size() == 0

Prefer container.empty() over container.size() == 0 for
clarity and consistency with the rest of the codebase.
empty() directly expresses intent and follows common
C++ container usage conventions.

Change-Id: Ia1beac941428d420b2c02a68cd66ea1732f9f989
Signed-off-by: Meghana Vangapandu <meghanav@ami.com>