PEL: Allow default AdditionalData construction
PEL: Allow default AdditionalData construction

Enable the default contructor so that an object can be created
without needing any data.

Also make getValue() const as it is const.

Signed-off-by: Matt Spinler <c000c989d08c98662d98f15448f11de169feb92d@us.ibm.com>
Change-Id: Id81879e0367edb1055cbf848ed738bc8dd86e953

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging