Fix JSON schema 'required' keyword parsing

:Release Notes:
Added checking of duplicate and empty values for "required" keyword
during JSON Schema parsing

:Detailed Notes:

:Testing Performed:
Unit tests with memcheck

:QA Notes:

:Issues Addressed:
[ANG-1452] pbnjson: Ensure the property "required" isn't an empty array

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

Change-Id: Id6f5149c19106457593e738205a30d6b2cae0565
Reviewed-on: https://g2g.palm.com/4624
Reviewed-by: DCO Verification
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Tested-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>