Fix luna-service2 crashes on invalid JSON inputs

:Release Notes:
Implemented additional checks for input JSON data in luna-service2
messages handling

:Detailed Notes:

:Testing Performed:
All unit tests passed

:QA Notes:

:Issues Addressed:
[GF-39303] luna-service2 cause crash if "subscribe" : null

Open-webOS-DCO-1.0-Signed-off-by: Mykola Kryvonos <mykola.kryvonos@lge.com>

Change-Id: Ic3019a73e2b4ca4e4f3f9e3d3c76e152a55dd7ba
Reviewed-on: https://g2g.palm.com/3569
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>