clientv3/sync: fix getting empty key error
clientv3/sync: fix getting empty key error

lumjjb/etcd,akauppi/etcd,sebrandon1/etcd,rnaveiras/etcd,joshix/etcd,lorneli/etcd,a-robinson/etcd,jlamillan/etcd,chipironcin/etcd,glycerine/etcd,zaphod-concur/etcd,westhood/etcd,coreos/etcd,rnaveiras/etcd,siddontang/etcd,etcd-io/etcd,westhood/etcd,purpleidea/etcd,etcd-io/etcd,mkumatag/etcd,jlamillan/etcd,achanda/etcd,visheshnp/etcd,dnaeon/etcd,hongchaodeng/etcd,bluebreezecf/etcd,joshix/etcd,ravigadde/etcd,li-ang/etcd,CrazyJvm/etcd,lorneli/etcd,OyTao/etcd_learning,YuleiXiao/etcd,xiang90/etcd,handsomesun/etcd,CrazyJvm/etcd,heyitsanthony/etcd,StorReduce/etcd,luxas/etcd,insequent/etcd,hongchaodeng/etcd,dannysauer/etcd,tamird/etcd,etcd-io/etcd,endocode/etcd,mkumatag/etcd,sinsharat/etcd,nekto0n/etcd,zaphod-concur/etcd,gyuho/etcd,xiang90/etcd,jonboulle/etcd,whuwxl/etcd,wantonsolutions/dinv-etcd,sebrandon1/etcd,siddontang/etcd,yudai/etcd,bluebreezecf/etcd,insequent/etcd,ajityagaty/etcd,StorReduce/etcd,sofuture/etcd,handsomesun/etcd,achanda/etcd,rnaveiras/etcd,dnaeon/etcd,lumjjb/etcd,qqsun8819/etcd,akauppi/etcd,bdarnell/etcd,cockroachdb/etcd,a-robinson/etcd,endocode/etcd,glevand/coreos--etcd,purpleidea/etcd,tguangch/etcd,swingbach/etcd,mqliang/etcd,tamird/etcd,mqliang/etcd,glycerine/etcd,dannysauer/etcd,ktateish/etcd,armstrongli/etcd,sofuture/etcd,bdarnell/etcd,armstrongli/etcd,cockroachdb/etcd,linuxcer/etcd,ktateish/etcd,nekto0n/etcd,mkumatag/etcd,vimalk78/etcd,gyuho/etcd,fanminshi/etcd,OyTao/etcd_learning,ravigadde/etcd,heyitsanthony/etcd,fanminshi/etcd,wantonsolutions/dinv-etcd,vimalk78/etcd,mqliang/etcd,rchicoli/etcd,mickep76/etcd,qqsun8819/etcd,luxas/etcd,plasticbox/etcd,visheshnp/etcd,ajityagaty/etcd,plasticbox/etcd,mickep76/etcd,linuxcer/etcd,glevand/coreos--etcd,tguangch/etcd,YuleiXiao/etcd,chipironcin/etcd,wantonsolutions/dinv-etcd,swingbach/etcd,yudai/etcd,coreos/etcd,ajityagaty/etcd,li-ang/etcd,whuwxl/etcd,jonboulle/etcd,sinsharat/etcd,rchicoli/etcd