mainline: return null session if sessionKey is null or empty when getting session by key
mainline: return null session if sessionKey is null or empty when getting session by key

ZixiangBoy/SuperSocket,jmptrader/SuperSocket,chucklu/SuperSocket,kerryjiang/SuperSocket,fryderykhuang/SuperSocket,mdavid/SuperSocket,memleaks/SuperSocket,fryderykhuang/SuperSocket,chucklu/SuperSocket,ZixiangBoy/SuperSocket,jmptrader/SuperSocket,fryderykhuang/SuperSocket,mdavid/SuperSocket,ZixiangBoy/SuperSocket,kerryjiang/SuperSocket,jmptrader/SuperSocket,mdavid/SuperSocket,chucklu/SuperSocket