C++ Avoid crash in case of Invalid/Empty Payload

C++ Avoid crash in case of Invalid/Empty Payload

This change is based on a bug reported by SHP wherein
they want to be able to send response with empty payload.
Instead of throwing an error, we now send an empty
OCRepresentation() object to the client callback.

Change-Id: I16f298bf8a0bdbb545e1133d751582f418991352
Signed-off-by: omkar <omkar.m.hegde@intel.com>
(cherry picked from commit 43e5a4f0a42a739e93f65ccf31eff82df11144c3)
Reviewed-on: https://gerrit.iotivity.org/gerrit/212
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>