mutation: make copy constructor compatible with mutation_opt
mutation: make copy constructor compatible with mutation_opt

Currently `_data` is assumed to be engaged by the copy constructor which
is not necessarily the case with `mutation_opt` objects (which is an
`optimized_optional<mutation>`). Fix this by only copying `_data` if
non-null.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla