db: don't flush empty memtables
db: don't flush empty memtables

You can't have an empty sstable, so don't flush an empty memtable.

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