Restconf often invokes RPCs with null payload

====Summary====
Title says it all - Restconf often calls RPC implementations with null payload.

====Examples====
One example of this is in SXP csit https://logs.opendaylight.org/releng/jenkins092/sxp-csit-1node-basic-all-nitrogen/125/
However I can easily reproduce this when running SXP basic robot tests on my local machine.

====Symptoms====

• Wireshark verified Robot sends valid requests with xml payload. (see attachment)
• Walking back through stackframes I saw that AbstractResourceMethodDispatchProvider (line 185) in jersey already holds params with null data
• However, http headers are parsed correctly

====Enviro====
Affects both Nitrogen and Oxygen.