v3rpc: treat nil txn request op as error
v3rpc: treat nil txn request op as error

Fixes #7889

heyitsanthony/etcd,bluebreezecf/etcd,fanminshi/etcd,armstrongli/etcd,sinsharat/etcd,xiang90/etcd,rnaveiras/etcd,cockroachdb/etcd,vimalk78/etcd,gyuho/etcd,bdarnell/etcd,bdarnell/etcd,visheshnp/etcd,cockroachdb/etcd,mqliang/etcd,coreos/etcd,a-robinson/etcd,fanminshi/etcd,ajityagaty/etcd,bluebreezecf/etcd,mqliang/etcd,etcd-io/etcd,mkumatag/etcd,ajityagaty/etcd,coreos/etcd,joshix/etcd,rnaveiras/etcd,rnaveiras/etcd,ajityagaty/etcd,mkumatag/etcd,li-ang/etcd,sinsharat/etcd,etcd-io/etcd,mickep76/etcd,glevand/coreos--etcd,akauppi/etcd,heyitsanthony/etcd,glevand/coreos--etcd,a-robinson/etcd,sebrandon1/etcd,YuleiXiao/etcd,mickep76/etcd,YuleiXiao/etcd,lorneli/etcd,akauppi/etcd,sebrandon1/etcd,li-ang/etcd,etcd-io/etcd,whuwxl/etcd,whuwxl/etcd,armstrongli/etcd,gyuho/etcd,xiang90/etcd,vimalk78/etcd,mqliang/etcd,visheshnp/etcd,dannysauer/etcd,lorneli/etcd,mkumatag/etcd,joshix/etcd,dannysauer/etcd