client: Replace nil value testing with assert.Nil
client: Replace nil value testing with assert.Nil

Multiple errors in previous commit where `nil` is not being detected by `assert.Equal`

mkumatag/etcd,rnaveiras/etcd,coreos/etcd,etcd-io/etcd,mkumatag/etcd,etcd-io/etcd,rnaveiras/etcd,gyuho/etcd,rnaveiras/etcd,mqliang/etcd,etcd-io/etcd,mqliang/etcd,mqliang/etcd,ajityagaty/etcd,ajityagaty/etcd,coreos/etcd,mkumatag/etcd,ajityagaty/etcd,gyuho/etcd