streaming: Improve handler of prepare_message from follower
streaming: Improve handler of prepare_message from follower

Upon receiving of the prepare_message message from the follower,
msg.requests can only be empty. Don't bother look at the requests.

rluta/scylla,phonkee/scylla,gwicke/scylla,rentongzhang/scylla,respu/scylla,rluta/scylla,victorbriz/scylla,shaunstanislaus/scylla,phonkee/scylla,eklitzke/scylla,tempbottle/scylla,linearregression/scylla,scylladb/scylla,senseb/scylla,rentongzhang/scylla,gwicke/scylla,avikivity/scylla,senseb/scylla,aruanruan/scylla,victorbriz/scylla,aruanruan/scylla,guiquanz/scylla,kangkot/scylla,phonkee/scylla,scylladb/scylla,gwicke/scylla,rluta/scylla,eklitzke/scylla,kangkot/scylla,guiquanz/scylla,duarten/scylla,tempbottle/scylla,respu/scylla,tempbottle/scylla,justintung/scylla,stamhe/scylla,avikivity/scylla,justintung/scylla,shaunstanislaus/scylla,glommer/scylla,acbellini/scylla,bowlofstew/scylla,rentongzhang/scylla,wildinto/scylla,duarten/scylla,asias/scylla,linearregression/scylla,kjniemi/scylla,dwdm/scylla,aruanruan/scylla,victorbriz/scylla,scylladb/scylla,raphaelsc/scylla,kangkot/scylla,kjniemi/scylla,dwdm/scylla,capturePointer/scylla,asias/scylla,asias/scylla,respu/scylla,eklitzke/scylla,glommer/scylla,acbellini/scylla,stamhe/scylla,raphaelsc/scylla,capturePointer/scylla,kjniemi/scylla,scylladb/scylla,shaunstanislaus/scylla,acbellini/scylla,avikivity/scylla,wildinto/scylla,bowlofstew/scylla,capturePointer/scylla,glommer/scylla,duarten/scylla,justintung/scylla,linearregression/scylla,wildinto/scylla,stamhe/scylla,bowlofstew/scylla,senseb/scylla,dwdm/scylla,raphaelsc/scylla,guiquanz/scylla