future: fix future<>::get0()
future: fix future<>::get0()

Empty tuples don't have a first element.

mixja/seastar,phonkee/scylla,dreamsxin/seastar,stamhe/seastar,eklitzke/scylla,chunshengster/seastar,kjniemi/scylla,senseb/scylla,glommer/scylla,dreamsxin/seastar,scylladb/seastar,gwicke/scylla,leejir/seastar,syuu1228/seastar,scylladb/scylla-seastar,linearregression/seastar,asias/scylla,dwdm/seastar,shaunstanislaus/scylla,scylladb/seastar,bowlofstew/scylla,duarten/scylla,acbellini/scylla,joerg84/seastar,wildinto/seastar,avikivity/seastar,shaunstanislaus/scylla,bowlofstew/seastar,hongliangzhao/seastar,guiquanz/scylla,duarten/scylla,rentongzhang/scylla,koolhazz/seastar,leejir/seastar,sjperkins/seastar,stamhe/seastar,aruanruan/scylla,flashbuckets/seastar,linearregression/scylla,avikivity/scylla,avikivity/scylla,tempbottle/seastar,chunshengster/seastar,kjniemi/seastar,bowlofstew/scylla,justintung/scylla,stamhe/seastar,senseb/scylla,xtao/seastar,capturePointer/scylla,anzihenry/seastar,glommer/scylla,shaunstanislaus/scylla,koolhazz/seastar,cloudius-systems/seastar,tempbottle/seastar,tempbottle/scylla,respu/scylla,linearregression/seastar,kjniemi/seastar,jonathanleang/seastar,stamhe/scylla,asias/scylla,mixja/seastar,kangkot/scylla,guiquanz/scylla,respu/scylla,printedheart/seastar,scylladb/seastar,dwdm/scylla,scylladb/scylla,bzero/seastar,sjperkins/seastar,linearregression/scylla,acbellini/scylla,wildinto/scylla,mixja/seastar,rentongzhang/scylla,stamhe/scylla,victorbriz/scylla,dwdm/seastar,kangkot/scylla,eklitzke/scylla,avikivity/seastar,kjniemi/scylla,xtao/seastar,kjniemi/seastar,rentongzhang/scylla,senseb/scylla,duarten/scylla,stamhe/scylla,kangkot/scylla,xtao/seastar,scylladb/scylla-seastar,anzihenry/seastar,tempbottle/scylla,respu/scylla,joerg84/seastar,asias/scylla,scylladb/scylla-seastar,guiquanz/scylla,scylladb/scylla,wildinto/scylla,dwdm/scylla,scylladb/scylla,glommer/scylla,avikivity/scylla,phonkee/scylla,flashbuckets/seastar,shyamalschandra/seastar,shyamalschandra/seastar,jonathanleang/seastar,bowlofstew/seastar,hongliangzhao/seastar,tempbottle/scylla,bowlofstew/scylla,aruanruan/scylla,joerg84/seastar,shyamalschandra/seastar,jonathanleang/seastar,acbellini/seastar,aruanruan/scylla,gwicke/scylla,koolhazz/seastar,dreamsxin/seastar,syuu1228/seastar,norcimo5/seastar,linearregression/seastar,acbellini/seastar,printedheart/seastar,hongliangzhao/seastar,tempbottle/seastar,avikivity/seastar,capturePointer/scylla,linearregression/scylla,bzero/seastar,wildinto/seastar,dwdm/scylla,acbellini/seastar,norcimo5/seastar,syuu1228/seastar,capturePointer/scylla,victorbriz/scylla,bzero/seastar,rluta/scylla,flashbuckets/seastar,cloudius-systems/seastar,raphaelsc/scylla,wildinto/scylla,printedheart/seastar,scylladb/scylla,justintung/scylla,anzihenry/seastar,rluta/scylla,raphaelsc/scylla,phonkee/scylla,kjniemi/scylla,cloudius-systems/seastar,sjperkins/seastar,bowlofstew/seastar,chunshengster/seastar,raphaelsc/scylla,justintung/scylla,norcimo5/seastar,wildinto/seastar,eklitzke/scylla,dwdm/seastar,leejir/seastar,acbellini/scylla,victorbriz/scylla,rluta/scylla,gwicke/scylla