Add a helper function to serialize `null'.
Add a helper function to serialize `null'.

Change-Id: If4373b3ae851c132e5eb64a563d0a49a7c37949f

StumbleUponArchive/asynchbase,OpenTSDB/asynchbase,nvoron23/asynchbase,angushe/asynchbase,manolama/asyncbigtable,yuzhu712/asynchbase,SteamShon/asynchbase,manolama/asynchbase,pythian/asynchbase,tempbottle/asynchbase,angushe/asynchbase,mway08/asynchbase,hhshuai/asynchbase,hzy001/asynchbase,manolama/asynchbase,tsuna/asynchbase,mway08/asynchbase,mapr/asynchbase,mapr/asynchbase,nvoron23/asynchbase,StumbleUponArchive/asynchbase,adityakishore/asynchbase,OpenTSDB/asyncbigtable,tempbottle/asynchbase,adityakishore/asynchbase,SteamShon/asynchbase,OpenTSDB/asynchbase,Connexity/asynchbase,hzy001/asynchbase,hhshuai/asynchbase,Connexity/asynchbase,yuzhu712/asynchbase,tsuna/asynchbase