etcdctl/ctlv3: allow empty key
etcdctl/ctlv3: allow empty key

Fix https://github.com/coreos/etcd/issues/5323.

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