mutation_partition: Make row::merge() tolerate empty row
mutation_partition: Make row::merge() tolerate empty row

The row may be empty and still have a set storage, in which case
rbegin() dereference is undefined behavior.

kjniemi/scylla,raphaelsc/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla